Fig. 7. Bacilli of anthrax. Bacilli from the spleen of a mouse inoculated with a culture. The bacilli were obtained from the blood of a cow which died of anthrax in Mississippi. The bacilli appear as rods stained blue. The round bodies are blood corpuscles, also stained artificially.

These words, however, are now wholly inadequate to express the complex processes of infection, and it may be said that each species of bacterium or protozoon has its own peculiar way of invading the animal body, differing more or less from all the rest. There are, however, a few broad distinctions which may be expressed with the help of these old terms. Infection, as laid down above, refers at present in a comprehensive way to all microorganisms capable of setting up disease in the body. Some microorganisms are transmitted directly from one animal to another, and the diseases produced may be called contagious. Among these are included pleuropneumonia, rinderpest, foot-and-mouth disease, rabies, cowpox, and tuberculosis. Again, certain organisms are perhaps never transmitted from one animal to another, but may come from the soil. Among these are tetanus, blackleg, anthrax to a large extent, and perhaps actinomycosis in part. These diseases, according to some authorities, may be called miasmatic. There is a third class of infectious diseases, the specific bacteria of which are transmitted from one animal to another, as with the contagious diseases, but the bacteria may, under certain favorable conditions, find food enough in the soil and in the surroundings of animals to multiply to some extent after they have left the sick animal and before they gain entrance into a healthy one.

This general classification is subject to change if we take other characteristics into consideration. Thus tuberculosis, because of its insidious beginning and slow course, would not by many be considered contagious in the sense that foot-and-mouth disease is; yet, in either case, the bacillus must come from preexisting disease. The disease of rabies, or hydrophobia, is not contagious in the sense that rinderpest is, because the virus of rabies must be inoculated into a wound before it can take effect; yet, in both cases, the virus passes without modification from one animal to another, though in different ways.

Again, all the diseases under the second group, which seem to come from the soil and from pastures, are in one sense contagious in that the virus may be taken from a sick animal and inoculated directly, with positive results, into a healthy animal. Other illustrations may be cited which show that these old terms are not in themselves satisfactory. There are so many conditions which enter into the process of infection that no single classification will give a sufficiently correct or comprehensive idea of it. These statements will be easily understood if the different infectious diseases in the following pages are studied with reference to the way or ways in which each disease may be contracted. Enough has been said, therefore, to show that if we wish to make ourselves acquainted with the dangers of any given disease, we must study it and not rely upon any single work to tell the whole story.

Infectious diseases have, as a general rule, a period of incubation, which comprises the time elapsing between the exposure to the infection and the actual appearance of the disease. This period varies with the malady. The most common symptom of this class of diseases is fever. The severity of the fever is measured by the temperature of the animal; this is readily and accurately ascertainable by the clinical thermometer. (See [Pl. III], fig. 1.) The other symptoms are variable and depend upon the particular organ or organs most implicated. Loss of appetite, cessation of rumination and milk secretion, and general dullness are symptoms quite invariably present in most infectious diseases.