BACTERIA.

Bacterium is the generic name given to the micro-organisms belonging to the schizomycetes, whether a bacillus (rod-shaped), coccus (rounded), spiral-formed (spirillum or vibrio), or filamentous (leptothrix and spirochœta). All these are destitute of chlorophyll and multiply by fission.[9] They are all extremely small, the width usually not exceeding 1 µ = 1 ∕ 25000 inch. Various names are given to them, which are synonymous, thus: germs,

When they cause disease they are called contagia. They multiply rapidly, and may reach maturity in 20 to 30 minutes. One bacterium may, under favourable conditions, become 16,000,000 in 24 hours.

Methods of Examination.—Until Koch discovered the method of cultivating bacteria on solid media, the science of bacteriology remained in its infancy, as it was impracticable to obtain pure, i.e. unmixed cultures of a given bacterium. Koch hit on the idea of mixing minute portions of cultivations of bacteria which were growing in liquid broth with liquefied gelatine, and then spreading the mixture on glass plates, and allowing it to solidify under cover, so that no atmospheric bacteria could contaminate the growth. When this was done, individual bacteria formed individual “colonies” scattered over the gelatine, and these could be identified by sub-culturing and other methods, for the details of which books on bacteriology must be consulted.

The food supply of most bacteria is vegetable or animal refuse. Some of them have a most useful purpose in nature, that of breaking down complex organic substances and reducing them to a simpler form. Thus bacteria play an essential part in purifying the soil, and in the operations in sewage tanks and on sewage farms (pages 192, 195 and 220). A thimbleful of ordinary garden soil which has received a periodical manurial dressing contains one to three million bacteria. Certain bacteria have been found to be capable of exercising an opposite effect, i.e. fixing the atmospheric nitrogen and building it up into the nitrogenous tissues of plants. Thus the nodules on the roots of leguminous plants consist of bacteria living in symbiosis with the protoplasm of the plant and supplying it with nitrogen in an assimilable condition. Pure cultures of these bacteria have been put on the market as nitragin, for enriching land poor in nitrogen. Thus a fairly complete cycle of nature is secured, and by rotation of plants (legumes alternating with other seeds), manures, especially nitrogenous manures, may be partially saved.

The souring of milk is caused by the bacillus lactis. This souring is an indispensable preliminary to the making of cheese, and the bacillus can now be used in pure culture to hasten the natural process. The peculiar aroma of good butter is due to a bacterium which has been isolated; and it can now be supplied in pure culture for butter-making, thus obviating bad butter.

Certain bacteria are disease-producing or pathogenic. The largest of these is the Bacillus of Anthrax, a disease common in sheep and oxen, and sometimes communicated to man. This bacillus is 1·2 µ thick and 6 to 8 µ long. When an animal dies of this disease it should be buried without cutting the skin. When exposed to the air this bacillus forms minute spores, very difficult to destroy. They may live for several years in pits in which animals dying from anthrax have been buried. Butchers have died when inoculated through cracked fingers when dressing the carcase of a cow which has had anthrax. Similarly men handling the hides of such animals may be inoculated, either with a form of disease in which rapid blood-poisoning is produced, or with a malignant carbuncle, from which recovery is possible if it be treated promptly. Wool sorters of mohair wool are very liable to suffer from a fatal form of pneumonia due to the dust from wool derived from animals which have died from anthrax (page [107]). This disease gives a good instance of possible attenuation of virus, of which another example is seen in small pox (page [293]). Pasteur grew anthrax bacilli in broth at a temperature of 110° Fahr. At this temperature the bacilli multiplied by division, and no spores were formed. By repeatedly sub-culturing after the bacilli had become old (i.e. by putting minute quantities of the growth into fresh broth) and exposing to air, he obtained anthrax bacilli which were only slightly virulent, only producing slight constitutional disturbance when inoculated, i.e. injected under the skin of sheep, and yet protected them against ordinary infection by anthrax. Other methods of attenuation of virus have been discovered. For instance the growth of the bacillus in the presence of a feeble antiseptic, or passing it through the circulation of an animal which is relatively insusceptible to the particular bacillus has this effect.

Other important pathogenic bacteria will be considered later (pages 298 to 398). It is only necessary here to mention that suppuration, erysipelas, puerperal fever, and a number of forms of blood-poisoning are due to the invasion of the system by cocci. A single round cell (commonly not more than 1 ∕ 25000 inch in diameter) is called a micrococcus. When in pairs as in the micro-organism causing pneumonia they are called diplococci; when in chains, streptococci (i.e. twisted); when in masses, staphylococci. When cocci and other micro-organisms are kept out of wounds, healing occurs without suppuration; this is the principle of the antiseptic and aseptic methods of treating wounds (pages 106 and 110). The question of immunity is discussed on page [288].

Saccharomycetes occur in fermenting substances, as in the fermentation of saccharine solutions. The only organism belonging to this order, which is associated with diseased conditions, is the Sarcina Ventriculi. This is found occasionally in the vomit or even in the urine of some persons.

The Zygophyta occur as thread-like growths, forming a mycelium. This is composed of jointed branching tubular cells, in which minute spores are produced. Each spore, when liberated from its tube, is capable of producing another mycelium, and thus the growth spreads. The spores may be carried through the atmosphere, thus producing infection at a distance. They have an average diameter of 6 µ = about 1 ∕ 4000 inch.

The following are the chief Zygophytous parasitic diseases:—Thrush is associated with the growth of a minute filamentous fungus, the oidium albicans. It is common in babies, who are improperly fed, and in old people, or in persons exhausted by any chronic disease. Small white patches collect on the tongue and neighbouring parts, and these are often followed by the formation of minute ulcers. When it occurs in children, the food must be carefully attended to, and feeding bottles frequently scalded, etc.

Ringworm is due to the growth of a large spored or a small spored fungus (known under the names of Microsporon Audouini; Trichophyton megalosporon endothrix, Trichophyton megalosporon ectothrix) which attacks the skin. It is most difficult to eradicate when it occurs in hairy parts, as the growth penetrates to the roots of the hairs, and continues to live here long after it has been destroyed on the general surface of the skin. The fungus spreads on the skin in gradually enlarging circles, forming rings with a slightly raised margin. It is extremely contagious, being especially apt to spread in schools. The spores may be carried about by means of hats or bonnets, by gloves, towels, razors, and other means. The disease often remains undetected for some time; and many cases, especially where the scalp is affected, remain contagious after they have been apparently cured.

The removal of ringworm, as of all other skin parasites, is effected by some local parasiticide. Prolonged treatment, including the pulling out of diseased hairs, is required for ringworm of the scalp. A special cap should be worn, when the patient mixes with others.

Favus, or “scald-head,” is due to the growth in the skin of a minute fungus called the Achorion Schönleinii, which invades the same parts as those affected by ringworm, but differs in its mode of formation of spores; yellow cupped discs from ¼; to 1 ∕ 3 inch in diameter being produced. It is very rare in England, and almost confined to persons (especially children) who are kept in a filthy condition. It is a common and fatal disease in mice. The treatment is similar to that of ringworm.

Tinea versicolor is caused by the growth in the epithelial cells of the skin, of a fungus called the microsporon furfur, which, unlike the two last, does not invade the hair or nails. It forms light brown patches covered with a horny scurf, which gradually spread, until nearly the whole trunk may be covered. It does not attack children, and never affects uncovered parts of the body. It chiefly occurs in those who do not take frequent baths, and who perspire freely. It can be removed by daily washing with soap and water and rubbing with a rough towel, followed by the application of a weak carbolic lotion.

Animal Parasites.—Animal parasites are found on the skin or in internal organs or in the blood or lymphatic vessels. The following are the most common:—

The Acarus Scabiei is a minute animal not unlike a cheese mite, which causes the disease known as scabies or the itch. It is probably never more than 1 ∕ 77 of an inch in length. The female has eight legs, with terminal suckers on the four front legs and hairs on the hind legs. The male is smaller than the female, and in the adult condition the two hindmost legs have suckers, as well as the four anterior. It remains on the surface of the skin, while the female burrows deeply in the substance of the epidermis. At the bottom of the oblique burrow it deposits ten to fifteen or more eggs, which hatch in a fortnight and then commence similar operations on their own account. Scabies generally starts between the fingers, whence it rapidly spreads. The disease is acquired from some patient suffering from the disease, or by contact with his apparel. It may become very severe when suspicion as to its parasitic character has not been entertained. Formerly it was called “the seven years’ itch,” from the great difficulty in curing it before its true cause was discovered.

The irritation caused by the insect produces eczema, and this may be thought to be the only disease present, unless careful examination is made for the burrows of the insect.

To remove this parasite, first the skin is softened, the superficial epidermis is removed, and the burrows are laid bare, by the daily use of hot baths with soft soap, and subsequent rubbing with flesh towels. Then some parasiticide, such as the well known sulphur ointment, is rubbed into all the affected parts of the skin. A few days’ perseverance in this treatment usually suffices for a cure. The patient’s clothes and bed clothes ought also to be thoroughly purified by boiling or by steam disinfection or by baking in an oven; otherwise he may become re-infected.

The Larvæ of several insects have been found embedded in the skin. In the ox, the larva or bot of the gadfly produces a troublesome disease, a large boil being formed under the skin as the larva grows. This larva has, on rare occasions, attacked human beings. Rare cases are recorded where other larvæ have become developed in men, in all upwards of twenty separate kinds of insects having been recognized. The treatment consists in removing the parasite.

The Chigoe, commonly known as the jigger or sand-flea, is a minute parasitic insect, found in the West Indies and northern parts of South America. It is so small as to be scarcely visible; but the impregnated female possesses a proboscis, by means of which it penetrates the skin generally near the nails and there develops a bladder the size of a pea, which sets up severe inflammation. To get rid of the intruder, the orifice by which it entered must be dilated with a needle, until large enough to admit of its extraction, without rupturing the cyst.

Several species of Fleas infest the human frame. They are propagated by means of eggs, the worms from which enclose themselves in a tiny cocoon before assuming the adult form.

Three varieties of Lice occur on the human skin. The first (pediculus capitis) infests the head, especially of children, and multiplies with astonishing rapidity, the female laying altogether about fifty eggs. The other two varieties are the body louse (pediculus corporis) and the crab louse (pediculus pubis).

Strict attention to cleanliness is the best means of getting rid of fleas and bugs. A wash made of carbolic acid and vinegar painted over bed crevices is very efficient. Lice may be removed from the head by cutting the hair short, and carefully cutting out any hairs to which nits are attached. The nits are cemented to the shafts of hairs. Washing the hair with methylated spirit or paraffin is also helpful in removing them. Afterwards the use of white precipitate ointment will prevent their re-appearance.

The Trematoda or Flukes furnish two human parasites, viz. the liver-fluke (Distoma hepatis), and the Bilharzia hæmatobia. The liver-fluke occasionally produces jaundice in man. In sheep it is the cause of the disease known as the “rot.” The Bilharzia hœmatobia is chiefly found in Egypt, and the Cape Colony. It is about a quarter of an inch long, and infests the blood vessels, more particularly of the kidneys; setting up severe irritation and the discharge of blood. It is probable that the eggs of this parasite are received in drinking water or on salads, though occasionally inoculation may occur through the skin when bathing.

The family of Nematoda possesses numerous parasitic members. The common thread worm (Oxyuris Vermicularis) is one of the most common of these. The female is 1 ∕ 3 to ½ inch in length, and inhabits chiefly the lower bowel. The ova, which are from 1 ∕ 490 to 1 ∕ 1100 inch in diameter, often gain access to drinking water, or are carried by flies, or received on salads, etc. The injection of salt and water into the bowel, and treatment tending to improve the general health, are the proper remedies.

The round worm (Ascaris Lumbricoides) inhabits chiefly the small intestine; hence medicines for its removal require to be given by the mouth. The female is from 10 to 14 inches long; the ova, of which each female discharges on an average 160,000 daily, are from 1 ∕ 340 to 1 ∕ 440 inch in diameter.

The whip-worm (Trichocephalus Dispar) is a smaller nematode, which is rarely met with in this country. The Dochmius Duodenalis is met with chiefly in Italy and Egypt. It sucks the blood in the intestine, causing dangerous anæmia. The Strongylus Gigas is chiefly found in the kidneys of the ox, dog, etc., and is very rare in man. It resembles a very large round worm. In the kidney it produces severe disorders. How it gets there is not known.

The Trichina Spiralis has been already described (page [23]).

The Filaria Dracunculus (Guinea Worm) seems to gain access into the stomach along with water, or possibly in some cases, by perforating the skin. It burrows among the tissues, especially of the legs, and attains a length of several feet. It causes large boils and sores, and through these the eggs escape and pass into water. Here the embryo which has escaped from the egg meets with a fresh water crustacean (cyclops), enters its body, undergoes larval growth, and is swallowed with its host by a man, in whom it burrows and undergoes its next stage of life.

The embryos of three species of Filaria infest the blood of man, chiefly in the tropics. One embryonic species is found in the blood of infested patients by day, one by night, and one during both day and night. The length of the embryos varies from 1 ∕ 75 to 1 ∕ 125 inch, and its width from 1 ∕ 3000 to 1 ∕ 3500 inch. The night embryo, which is the most common, is produced by the Filaria Bancrofti. This adult worm infests the lymphatic system of man, sometimes reaching a length of three to four inches. Its embryos may obstruct lymphatic vessels, causing obstruction of the flow of chyle (hence originates chyluria), and elephantiasis, in which enormous swelling of the legs and other parts ensues.

The nocturnal migration into the lymphatic vessels, and thence into the blood of the embryo of the F. Bancrofti, is an adaptation to the nocturnal habits of a particular mosquito (culex pipiens or ciliaris). When the mosquito bites an infested person, his proboscis removes some embryo filariæ, which are quickly transferred to its stomach. Some of these escape digestion, develop within the mosquito, and when the mosquito dies in water they bore their way out, and are subsequently swallowed by man.

It is essential, therefore, in order to prevent this disease to boil or efficiently filter all drinking water, and to prevent the access of mosquitoes to water. Persons infested with filariæ should sleep inside mosquito nets, in order that they may not, when bitten by mosquitoes, spread the disease.

Tape-worms are found infesting the alimentary canal of man. Each has a double phase of existence. In the first, the characteristic head, or scolex, along with a bladder-like body, lies embedded in the solid tissues of an animal; in the second, the strobilus or tape-worm, occupies the alimentary canal of another animal. The tape-worm consists of a number of flat segments, each of which is capable of producing a large number of eggs, from each of which a six-hooked embryo is developed. The segments escape from the alimentary canal, and their ova are discharged and scattered broad-cast. These eggs are swallowed by another animal, the hooked embryo escapes from its case, migrates into the solid tissues, and there produces a scolex. When the host is eaten by another animal or by man, the scolex enters the alimentary canal, loses its bladder-like body, and developes a chain of segments. It follows from the above that two distinct hosts are necessary to complete the cycle of existence of these creatures, one being commonly a herbivorous, and the other a carnivorous animal. Thus:—

Cystic Form. Tape-worm Form.
Cysticercus Cellulosæ in the muscles of the pigbecomes Tænia Soluim in the alimentary canal of man.
Cysticercus Bovis in the muscles of the ox„Tænia Mediocanellata in the alimentary canal of man.
Cænurus Cerebralis of the sheep’s brain„Tænia Cænurus in the alimentary canal of the dog.
Echinococcus of man, etc.„Tænia Echinococcus in the alimentary canal of the dog.

The cysticercus cellulosæ has been already described (page [23]). The cystic form of the dog’s tape-worm (echinococcus) is a most dangerous parasite for man. When the egg of the dog’s tape-worm is swallowed by man, the embryo escaping from this egg burrows from the alimentary canal, and forms large cysts, chiefly in the liver, but occasionally in the lungs, brain, and other organs. For the removal of these, surgical interference is required. This form of cyst is commonly known as a hydatid. It is most frequently seen in Iceland and Australia, though not uncommon in this country. Its frequency depends largely on the number of dogs, and on the facility with which the ova of their tape-worms can gain access to water.

The adult Tape-worms are usually derived in man from eating meat containing the cystic form. The cysticercus of the pig produces Tænia Solium; that of the ox, the Tænia Mediocanellata.

These are the two most common forms of tape-worm in man. The minute head of T. Solium has four suckers and a double row of hooklets, 28 in number; while the head of T. Mediocanellata has four suckers but no hooklets. The segments of T. Solium are smaller than of T. Medioc., and the structure of the segments of the two is somewhat different.

Preventive Measures.—In avoiding the various Entozoa described, it is important (1) to carefully avoid all underdone meat. The eating of smoked sausages, or of meat which is not cooked throughout, is a common source of tape-worm and of trichinosis.

(2) All vegetables should be thoroughly washed: this is especially important in the case of water-cress, lettuce, etc., which are eaten raw.

(3) If the purity of the water is not ensured, it should be boiled or filtered through a Pasteur-Chamberland filter (page [98]), especially in tropical climates, and where many dogs are kept. Dogs should be kept out of the kitchen, lest ova accidentally gain access to articles of food.

(4) The possibility of flies and mosquitoes acting as carriers of parasitic disease must be remembered, and precautions taken.

[CHAPTER XLII.]
THE RÔLE OF INSECTS IN SPREADING DISEASE.

Insects are now known to be important agents, (a) as carriers and (b) as intermediate hosts of disease-agents.

The common domestic fly (Musca domestica) is the unwelcome companion of man in nearly every country. The eggs are usually laid and the larvæ undergo their development in excrement, but the female sometimes selects meal, bread, or fruit for the purpose. In practice, however, one of the best means of diminishing the number of domestic flies is to insist on the daily removal of all manure, especially horse manure, and to sprinkle the manure receptacle in the interval with lime. The fly may obviously be the means of conveying infected material from place to place. Anthrax has been ascribed to this cause. Nuttall has proved experimentally that flies are able to carry the infection of plague, and that they die of the disease. The presence of enormous numbers of flies in cholera times has been noted. Experimentally, flies caught in cholera wards have been found to harbour the cholera spirillum. It is probable that they play a serious rôle in spreading the infection of cholera. Hence all infectious dejecta (stools and urine) should be covered until finally disposed of, and food should be protected against flies. The same remarks apply for enteric fever. Flies fed with pure cultures of the bacillus of enteric fever pass these bacilli in their dejecta in a still virulent condition. In camps, especially in connection with large armies, there is the strongest reason for believing that flies carry infection from latrines to food. Flies have been known to feed on the expectoration of consumptive patients and it is possible therefore that they may thus infect food.

The bed bug (Cimex lectularius) has been stated to be capable of conveying by its bite the infection of plague and other diseases from an infected to a healthy person; but Nuttall’s experimental results were entirely negative.

Fleas (pulex) probably do not play any part in spreading anthrax. Experimentally, anthrax bacilli die off rapidly in fleas. In India, persons who had handled rats dead of plague frequently acquired the disease. This was explained by Simond on the supposition that the fleas abandoned the dead rat for the human subject. The rats which appeared to have caused plague in man were stated to have died but a short time before; and the handling on the day after their death of rats dead of plague was stated to be safe because the rats’ fleas had then deserted the dead rat. It is assumed that the flea injects the poison of plague under the skin. On the contrary it is to be remembered that the fleas infesting rats and mice belong to a different family from that which attacks man. Whether this is a usual means of conveying plague may therefore be regarded as still doubtful. That rats convey plague to man is certain; whether fleas act as an intermediary remains somewhat uncertain.

The Mosquito family (Culicidæ) has been found to be an important if not the sole means of spreading certain serious diseases to man. To this family belong all true gnats or mosquitoes; but the only two genera which have been proved to be able to cause disease are Culex and Anopheles. The culex may usually be distinguished by the fact that when alive and at rest its head is below the level of the thorax and abdomen, thus giving the insect a hump-backed appearance, while the body of the anopheles under the same circumstances is all in a line.[10] The anopheles is more slender and its head smaller than that of the culex. The anopheles usually confines its blood-sucking operations to the evening and night. During the day it remains in dark corners. It lays its eggs usually in a natural pool or pond on the ground, on the surface of the water. In about two days a minute larva is hatched out. This grows rapidly, assumes the pupa form, from which the perfect insect emerges. The female insect alone is blood-sucking. In about 20 days after birth, it lays from 150 to 200 eggs. Its relation to malaria may be gathered from the following historical sketch. In 1880 Laveran found in the red blood corpuscles of malarious patients minute bodies which he regarded as not bacteria, but a very low form of animal life, possessing amœboid movements. These grew at the expense of the blood corpuscles, deposited a dark pigment, and often assumed the appearance of a “rosace,” a rounded body with little spherules at its circumference. Golgi in 1889 observed differences between the rosaces of tertian and quartan fever, and found that the periods of occurrences of the fever corresponded with the times of maturation of the rosaces. It was concluded therefore, that the rosaces caused the fever by shedding their sporules into the blood. These sporules when thus shed were found to attach themselves to, and grow in, other red blood corpuscles. It is now known that there are three species of the parasite, in one of which the parasites are crescentic in shape. The examination of a drop of blood from a patient now enables a doctor to recognise which of these three forms of malaria he is dealing with.

Laveran observed that certain forms of the parasite presented “flagella,” i.e. filaments exhibiting very active movements. Manson having observed that flagella were not found in blood first drawn, but only appeared after a little time had elapsed, conceived the idea that the function of these must be that of spores. Having previously observed that a microscopic worm, filaria, is drawn with the blood into the stomach of a kind of mosquito (page [278]), and finds in the latter a secondary host, he concluded that a similar cycle of events might occur in malaria. Ross tested this theory, and by causing mosquitoes bred in bottles from the larva to bite persons affected with the crescent form of malaria, after repeated unsuccessful attempts, was eventually able to find in comparatively rare mosquitoes which had thus bitten a malarious patient, small rounded bodies embedded in the wall of the stomach. These were watched and found to present appearances identical with those of the parasite of malaria. Similar pigmented bodies were subsequently found in other mosquitoes.

The malarial parasite belongs to the Protozoa, of which it is one of the smallest members. Man is its intermediate host, and the anopheles its definitive or final host. In the red blood corpuscle of man it is a unicellar organism, from 1 µ to 8 µ in diameter. It has two methods of reproduction, endogenous by spore formation and exogenous or sexual. The former occurs in man; the latter in the mosquito. Without the latter, the parasite being unable to pass from man to man, would die with its host. In endogenous multiplication spores are formed which separate from the original parasite and gain access to other red blood corpuscles. The large pigmented spheres and the crescent bodies require to enter the stomach of the anopheles to attain full development. In the anopheles the crescents become spherical, flagella are shot out, having a length of 4 to 5 times the diameter of a red blood corpuscle. These represent the male element, while other spheres without flagella are the females. By the fusion of these two a fertilised cell is produced (the travelling vermicule), which now assumes the shape of a spear-head and is actively mobile. The travelling vermicule pierces the stomach wall of the mosquito and develops into a zygote. If an infected mosquito is examined on a succession of days under the microscope, the following stages can be traced. The zygote consists of pigmented spheres 7 to 8 µ in diameter, lying in the muscular fibres of the mosquito. These grow, and become surrounded by a capsule. Smaller spheres form and subdivide, bud-like processes develop on their surfaces; these gradually become sickle-shaped and protrude into the body cavity. They increase in size. until they attain dimensions of from 40, to 60 µ. Eventually they rupture, and the sickle-shaped bodies (sporozooites) escape and are carried in the body fluid of the mosquito to its salivary glands. These sporozooites are about 14 µ long, and human infection is caused by them. They have been traced as far as the end of the proboscis of the mosquito. (See also page [307]).

[CHAPTER XLIII.]
INFECTIVE DISEASES.

The prevention of disease depends largely on a knowledge of its causes. Disease may be due to a personal life not in accordance with physiological laws; or to some cause or causes acting ab extra. With advance of knowledge the number of diseases which can be proved to be caused by a contagium vivum introduced from without is steadily increasing. We have already discussed the influence of habits, of clothing, exercise, sleep, and food on health, and have shown how errors in these respects may lead to disease. It now remains to consider more particularly the prevention of diseases, due to the introduction into the system of contagia.

In the study of such diseases three chief factors require consideration: (1) the contagium itself; (2) conditions of environment, as climate, soil, season, weather, etc., which may favour or impede its spread; and (3) personal conditions which similarly influence it. Of these age, heredity, fatigue, injury, diet, and race are specially important.

The first two groups of diseases given in the Registrar-General’s classification of causes of death are (1) Specific Febrile or Zymotic Diseases, and (2) Parasitic Diseases. The objection to the word “specific” is that, although in most instances diseases in this group are “specific” in the sense that they are caused by a particular microbe, e.g. tetanus, anthrax, tuberculosis, in a few instances the same lesions may be caused by several microbes, e.g. septicaemia (blood-poisoning), pneumonia. “Zymotic” was the name given by Farr, in view of the analogy of the febrile process to that of alcoholic fermentation. In both there is the introduction of a living germ or germs; in both a period of “incubation” in which nothing can be observed; then follows the active disturbance; and in the disease, as well as in the fermenting liquid, the process is stopped, when the microbes have multiplied to a certain extent, a temporary or permanent protection being the result. The best name for the diseases in this group is “Infective.” Parasitic diseases, like ringworm, scabies, or trichinosis, are also infective; but for convenience may be described separately as “parasitic.”

The relation between the words “infectious” and “contagious” requires explanation. A disease like measles or small-pox, which can be transmitted from person to person, without immediate contact between the two, is termed infectious. In these cases the infection is conveyed by mucus expectorated or by dust blown about, or carried in apparel, etc., from the first patient. Such diseases may also, of course, be communicated by direct contact. If direct contact between the sick and well is indispensable for the transmission of a disease it is called contagious. There is no such hard line in nature, although some diseases can be more easily communicated than others. The term contagious is usually applied to parasitic diseases like ringworm and scabies, but even these can be communicated by means of infected articles as well as persons. The word contagious should be abandoned for all the acute febrile diseases. The word infective is used to include all specific febrile diseases, however spread. This word, therefore, includes not only infectious and contagious diseases, but also diseases spread by inoculation, i.e. injection of the infection under the skin. Thus malaria is not infectious from patient to patient; but can be inoculated by the mosquito.

Infective Diseases are either acute or chronic. Of acute infective diseases small-pox and enteric fever are typical examples; of chronic, tuberculosis and syphilis.

It was formerly supposed that in certain diseases the contagium or infective agent grew in external noxious matter, a miasm being produced; while in other diseases contagion was only produced direct from patient to patient; and others originated in either way. Hence the classification of infective diseases into (a) miasmatic, (b) contagious, and (c) miasmatico-contagious diseases. This classification has now been abandoned. Thus influenza and ague were formerly thought to be miasmatic; but the former is spread by personal infection; the latter by inoculation of the contagium by an infected mosquito.

Bacteriology has thrown an immense light on the causation of infective diseases. A large number of these have been proved to be caused by bacteria, and by analogy we infer the same thing for many others. Koch has laid down the following postulates as necessary before it can be stated that a particular disease is directly caused by a given microbe:—

(1) The microbe shall be demonstrated in the diseased tissues or blood of man or an animal suffering or dead from the disease.

(2) The microbes shall be isolated from these and cultivated in suitable media until obtained in pure culture. That is to say, matter containing the microbe, taken from the infected source, must be cultivated in artificial media outside the animal body, under conditions excluding the possibility of the introduction of other microbes, until pure cultures of these microbes are obtained, and these microbes must be transplanted from generation to generation, until it is certain that no trace of non-living matter derived from the original animal body remains in the culture.

(3) A pure culture of the microbe, thus obtained, shall, when introduced into the body of a healthy susceptible animal, reproduce the disease in question.

(4) The microbe in question shall be found in the animal so affected. Kanthack adds a further condition, that

(5) The toxins and poisonous substances obtained from the artificial cultivations shall agree chemically and physiologically with those obtained from the diseased animal.

All the preceding conditions have been fulfilled for anthrax, diphtheria, and tetanus; and the first four conditions have been fulfilled in regard to tuberculosis, glanders, gonorrhœa, malignant œdema, and actinomycosis. In enteric fever and influenza the first two conditions have been met; but inoculation experiments (3) have failed. In leprosy and relapsing fever the first condition is met, but (2) has failed.

In the following diseases the specific microbe has not been isolated, though from analogy it is believed that each of them is caused by such a microbe:—

Erysipelas occupies a special position. It is a specific disease due to a microbe, which, when it attacks other parts than the skin, may produce abscesses, boils, or blood-poisoning.

Bacteria are either saprophytes, i.e. they can grow on dead organic or even inorganic matter; or parasites, i.e. they are dependent for their existence on a living plant or animal which they invade. There are two varieties of each of these, obligate and facultative. An obligate parasite can develop only within a living host; while a facultative parasite can, according to circumstances, lead either a parasitic or saprophytic form of existence. The fact that certain contagia are completely, and others only partially, parasitic brings out important differences in their life-history. Thus, so far as we know, the contagia of scarlet fever, measles, small-pox and hydrophobia do not multiply outside the body. Hence there is a reasonable prospect of annihilating them by measures of disinfection and isolation. The position of diphtheria is doubtful. It may have a saprophytic phase of life. The contagium of tuberculosis, as well as of erysipelas, may have a life outside the host, though to what extent is doubtful. Cholera and enteric fever, although generally communicated by infection, appear sometimes to be communicated by contagia grown in saprophytic life, remote from preceding cases.

The infection caused by bacteria may be local or general. Thus in tetanus and in diphtheria the invading bacteria usually remain at their original point of invasion (under the skin in tetanus, in the throat in diphtheria). In anthrax always, and often in enteric fever, they are present in the general circulation. In both instances the symptoms of disease are due chiefly to the toxic products or toxins formed by the bacteria. These toxins are enzymes, ptomaines, tox-albumins, etc. The specific toxins of anthrax, diphtheria, and tetanus have been identified; and by this means the possibility of neutralising them is created.

The channels of infection, i.e. of invasion of contagia, are the skin and the mucous membranes, particularly of the digestive and respiratory tracts.

The Incubation Period of an infectious disease is the interval elapsing between the receipt of infection and the earliest development of symptoms. The period of incubation of the chief infectious diseases is shown in the following table:—

DISEASE.BEGINS USUALLY
ON THE		BUT MAY POSSIBLY BE AT
ANY PERIOD BETWEEN
Scarlet fever4th day. 1 and 7 days.
Diphtheria2nd day. 2 and 5 days.
Small-pox12th day. 1 and 14 days.
Chicken pox14th day. 10 and 18 days.
Typhus fever12th day. 1 and 21 days.
Enteric fever14th-21st day. 1 and 28 days.
Cholera1st-3rd day. A few hours and 10 days.
Measles12th-14th day. 10 and 14 days.
Rötheln (German measles)14th day. 12 and 18 days.
Mumps19th day. 16 and 24 days.
Whooping cough14th day. 7 and 14 days.
Influenza2nd day. 2 and 6 days.

The period of incubation is several weeks in hydrophobia and syphilis, and may be several years in leprosy.

Following the period of incubation, come the premonitory symptoms, which usually are somewhat sudden in onset. For the chief symptoms of onset see page [318].

Persons vary in susceptibility to attack by different infective diseases. The intensity of an attack depends on the condition of the patient, and on the number and the virulence of the particular microbes infecting the patient. In certain families attacks of particular diseases are more severe, and attacks are more liable to occur than in others.

It has been shown in certain diseases that the cells and the fluids of the body have a protective effect against infection. This protective action varies in different persons, and in the same person at different times. The cells of the body (phagocytes) swallow up and destroy a certain number of bacteria. This action is called phagocytosis. It is overcome when the dose of contagium is excessive, or when the vitality of the individual is lowered, especially the local vitality at the part attacked. Thus children with “weak throats” are particularly prone to scarlet fever and diphtheria.

The protection afforded by one attack of an infective disease against its recurrence varies greatly. A second attack of small-pox is very rare, of scarlet fever less uncommon, of diphtheria common. In erysipelas, influenza, pneumonia, and rheumatic fever, second or even more numerous attacks are common.

Immunity against an infective disease may be natural, but is more often acquired by an attack of the disease in question. This latter immunity is active, and is due to the formation in the tissues of the immunised person or animal of substances produced by the reaction of these tissues to the stimulus of the contagium. Thus a pig when it has recovered from an attack of swine-plague has produced what are called in German antikörpers, and its tissues are now a medium unfavourable to the growth of the bacillus of swine-plague. If the serum of the protected pig is injected under the skin of another pig, the latter acquires passive immunity against swine-plague, which is not so persistent as active immunity.

Active Immunity can be produced (1) by an attack of an infective disease, or (2) by artificial inoculation (under the skin) of the contagium of the disease, producing a milder attack of the disease. This may be done (a) by inoculating small doses of a virulent contagium, as in the inoculation of small-pox from a previous patient; or (b) by inoculating an attenuated virus, as in vaccination. Inoculation of small-pox virus usually produced a milder attack than infection by ordinary means; but patients thus inoculated were a great source of danger to other persons. In vaccination the virus of small-pox is employed, which has become attenuated by passing through the calf. In its passage, it has lost the power of producing anything beyond a vesicle at the point of inoculation. The principle of protecting by attenuated virus was extended by Pasteur, who was able to render animals resistant against anthrax, swine-fever, and quarter-evil, and hens against fowl-cholera, by inoculating them with attenuated cultures of the contagia of these diseases. Haffkine has applied the same method on a large scale for cholera.

The above are methods of bacterial vaccination. Salmon and Smith have shown that artificial active immunity can be produced also by (3) toxin-injection. They artificially cultivated the hog-cholera bacillus in broth. This broth was then sterilized, the bacilli being killed, but their products remaining. By injecting pigeons with this sterilized broth they made them resistant to subsequent infection by the bacillus itself, thus proving that immunity can be produced by chemical as well as by biological means. The immunity was proportional to the dose of the toxin absorbed. By gradually increasing the dose, it was found practicable to confer immunity, not only against doses of toxin that would otherwise have been fatal, but also against bacterial infection by the particular bacillus used in manufacturing the toxin.

Passive Immunity.—Behring and Kitasato found that if the toxin (free from the bacilli) of tetanus be injected into an animal in increasing doses until it becomes immune against infection by the bacilli of tetanus, the blood serum of the animal in question injected into white mice confers the same immunity on them. The protection thus conferred is only temporary. Exactly the same procedure has been adopted for diphtheria, and it is now found that by injecting anti-diphtheritic serum into children who are exposed to the infection of diphtheria, they can for several weeks be prevented from developing the disease. This is of great practical importance, as meanwhile the source of infection can have been removed. Furthermore, the protective serum is also curative, and by its means diphtheria, if early treated, can be reduced from a dangerous to an insignificant disease.

Various theories have been propounded to explain immunity. Pasteur supposed that the special pabulum or food of the bacillus of the given disease became exhausted; but this does not fit in with the immunity that can be produced by toxins and anti-toxins. Chauveau supposed that certain bacterial products are retained in the body, rendering it unsuitable for further growth of the particular bacillus; just as more than 14 per cent. of alcohol in a saccharine solution prevents further fermentation. This does not explain all the facts. Metschnikoff concluded that the fight of the leucocytes and phagocytes of the body against weaker bacilli, gave the power of fighting and overcoming a more virulent bacilli, and that these properties of the cells were transmitted to later generations of body cells. This theory fails to explain the acquired immunity against toxins as well as against bacteria which occurs. The discovery that the blood and other normal tissue fluids possess some power of destroying bacilli has relegated the phagocytal theory to a secondary position.

Natural Immunity varies in different animals. Thus enteric fever, scarlet fever, and measles are not known to occur except in man. Tuberculosis, anthrax, hydrophobia (called rabies in the dog), glanders and tetanus are common to man and certain other animals. Man, cattle and pigs frequently suffer from tuberculosis; goats, sheep, horses, and dogs are practically immune to it.

Epidemic and Endemic Diseases.—Infective diseases may occur sporadically, in epidemics, or in pandemics, i.e. epidemics spread over a number of countries. The word epidemic is used here to mean specially prevalent, and not to apply only to infective diseases. Thus there may be an epidemic of arsenical poisoning from contaminated beer.

Certain infective diseases are endemic or topical, i.e. they have special homes or centres, from which they occasionally spread as epidemics. Yellow fever, cholera, and malaria belong to this group. In a minor degree enteric fever, epidemic diarrhœa, and tuberculosis may be described as endemic.

Each infective disease has a special seasonal incidence. Of these the most important are the autumnal group, viz.

Epidemic Diarrhœa,maximumprevalenceinJuly and August.
Enteric Fever„„„November, but excessive, Aug. to Dec.
Erysipelas„„„Nov. to Dec.
Diphtheria„„„Nov. and Dec., excessive, Sept. to Dec.
Scarlet Fever„„„Oct., excessive in Aug. to Dec.
Of other infective diseases
Small-pox has its„„„May, but is excessive Jan. to June.
Whooping Cough„„„Dec. to May.
Measles commonly has two seasonal maxima, in June and December with intervening minima.

Causes of Epidemics.—Measles recurs in the large towns of England every alternate year. Other infective diseases occur at less regular intervals. The recurrence of epidemics is not solely due to personal infection and the accumulation of a population at susceptible ages. There are longer cycles of the causes of which but little is known. Thus scarlet fever has been shown by Longstaff and Gresswell to become epidemic chiefly in dry years; and I have shown that diphtheria and rheumatic fever become widely epidemic under the same conditions, diphtheria becoming so only when a series of dry years occur in immediate succession.

[CHAPTER XLIV.]
ACUTE INFECTIVE DISEASES.

Acute Infectious Diseases are characterised by certain definite characters.

1.—They are usually infectious or contagious. It is preferable to use these two terms as interchangeable. The modes in which infection is received vary greatly with different fevers.

(1) Some can only be propagated by inoculation—the introduction through an abraded surface of a minute quantity of the poison; as in glanders and hydrophobia. Others, again, may be introduced in this way, but are usually acquired in another manner, as scarlet fever, small-pox.

(2) Some are carried through the atmosphere. The contagium of small-pox can be carried as far as any, while that of typhus fever only traverses a few feet. The atmosphere acts as a conveyer of infection, and the infectious matter must necessarily, in most instances, be in the condition of dust to enable it to be wafted by currents of air or disturbed by the movements of persons in an infected room.

(3) Clothes, books, and furniture are not uncommonly carriers of infection. An old letter, or a lock of hair, has even after many years’ concealment in an enclosed space produced infection on being brought to light. Woollen articles convey infection more easily than calico, and dark clothes better than light coloured. A fever nurse’s clothes should never be woollen, but some washable material.

(4) Drinking water and food often form a vehicle for infection. Milk and water are the two usual sources of infection; but uncooked food, especially oysters and mussels, fed in sewage-polluted estuaries, may produce the same effect. Cholera, enteric fever, dysentery, and summer diarrhœa are the chief diseases from this source; but scarlet fever and diphtheria occasionally have a similar origin. Milk may be infected from having been handed by an infectious patient; or it may possibly convey infection of the disease from which the cow at the time is suffering, e.g., tuberculosis (see also page [312]). Water may be contaminated with sewage or the excreta of a single infectious patient.

2. They retain their specific character and origin. Small-pox never produces scarlet fever, nor vice versâ; and it is found universally that all the specific fevers “breed true,” each one retaining its identity. More than this, a previous case of the same fever can nearly always be detected on careful examination. Overcrowding and other insanitary conditions diminish the resistance to infection, and may increase its virulence.

3. The behaviour of contagia, when received into the system, is characteristic of these diseases. There is first of all a period of latency or incubation, during which no symptoms are manifested (see page [287].) The incubation period is followed by the characteristic symptoms of the particular fever, which disappear in a variable period, leaving the patient, as a rule, more or less insusceptible to a second attack (see page [288]).

Throughout the progress of the disease, except in the period of incubation, the patient is able to communicate his disease to persons about him who have not been rendered safe by a previous attack. The way in which he thus communicates his disease varies in different cases. In scarlet fever, the throat and skin are the chief sources of contagion; in influenza, whooping-cough, and measles, the secretions from the respiratory passages; in hydrophobia, the saliva; in enteric fever and cholera, the vomit and stools.

Prevention of the Spread of the Chief Acute Infectious Diseases.—We may divide these into three classes. (1) Those which are infectious by contact with the patient or by the atmosphere around him. (2) Those in which the intestinal and renal evacuations are almost alone infectious; as enteric fever and cholera. (3) Those in which inoculation through an abraded surface is generally if not always necessary to produce infection.