Infection. The grounds for believing that each of the large class of communicable diseases depends upon the presence within the body of a distinct living organism have already been stated (see GERM). The manner in which each of these supposed organisms behaves in originating fresh cases of disease is, however, almost as characteristic as the effects it produces on the body.
(1) In malarial or miasmatic diseases, chief among which is ague, though they present many analogies to truly infectious diseases, there is no evidence that the malady can be transmitted from the sick to the healthy. The disease poison is derived from soil, water, or air, in which it seems to live and multiply.
(2) Intermediate between these and the more characteristic infectious diseases is a group of which cholera and typhoid (enteric) fever may be taken as types. Here the infectious material has its origin chiefly from the dejecta of the patient, but seems to acquire infectious properties only after it has been some time (probably for several days) outside the human body.
(3) The largest and most typical class includes typhus, smallpox, measles, scarlet fever, hooping-cough, and many others. In all these the disease is directly and immediately communicable from the sick to the healthy. But there are striking differences in the conditions under which infection usually takes place. The poison of typhus, the dreaded 'jail fever' of past times, is rapidly destroyed by admixture with air, and the danger of its spreading can be much diminished by free ventilation. In smallpox the infection can retain its vitality for years on the walls of a room, or in the artificially dried discharge from the pustules; in scarlet fever it may exist for many months in articles of clothing. Measles is not least infectious in the early stage, when it presents merely the symptoms of a bad cold; scarlet fever infection is not at its worst till the rash has faded and the skin begins to be shed.
(4) The last group consists of those diseases in which the poison does not diffuse itself through the air, but requires to be directly inoculated to produce the disease—e.g. syphilis and hydrophobia.
This classification of diseases believed to be dependent upon organisms, though practically convenient, cannot be considered a strictly accurate one; for many of the diseases in group 3, perhaps all, can be propagated by inoculation, and the infection of some may be able to develop outside the body and behave like those in group 2. Enough has been said to show the complexity of the problems, both practical and scientific, presented by the subject. As to the Infectious Diseases Notification Act of 1889, and other cognate matters, see HYGIENE, DISINFECTANTS, CONTAGION, and the articles on the several diseases.