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.