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 fever | 4th day. | 1 and 7 days. |
| Diphtheria | 2nd day. | 2 and 5 days. |
| Small-pox | 12th day. | 1 and 14 days. |
| Chicken pox | 14th day. | 10 and 18 days. |
| Typhus fever | 12th day. | 1 and 21 days. |
| Enteric fever | 14th-21st day. | 1 and 28 days. |
| Cholera | 1st-3rd day. | A few hours and 10 days. |
| Measles | 12th-14th day. | 10 and 14 days. |
| Rötheln (German measles) | 14th day. | 12 and 18 days. |
| Mumps | 19th day. | 16 and 24 days. |
| Whooping cough | 14th day. | 7 and 14 days. |
| Influenza | 2nd 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.