I. In or on inanimate objects or material. (_a_) As true saprophytes. (_b_) As facultative saprophytes. (_c_) Though obligate parasites, they exist in a latent state. II. In or on other animals, or products from them: A. Susceptible to the disease. (_a_) Sick themselves. (As far as human beings are concerned these are mainly: 1. Other human beings for most diseases. 2. Rats for plague. 3. Dogs for rabies. 4. Horses for glanders. 5. Cattle, swine, parrots for tuberculosis). (_b_) Recovered from illness. (_c_) Never sick but “carriers.” B. Not susceptible. (_d_) Accidental carriers. (_e_) Serving as necessary intermediate hosts for certain stages of the parasite--this applies to _protozoal_ diseases only, as yet. I. (_a_) The bacilli of tetanus, malignant edema and the organisms of “gas gangrene” are widely distributed. There is no evidence that their entrance into the body is at all necessary for the continuation of their life processes, or that one case of either of these diseases ever has any connection with any other case; they are true saprophytes. Manifestly it would be futile to attempt to prevent or eradicate such diseases by attacking the organism in its natural habitat. _Clostridium botulinum_, which causes a type of food poisoning in man, does not even multiply in the body, but the disease symptoms are due to a soluble toxin which is produced during its growth outside the body. (_b_) Organisms like the bacterium of anthrax and the bacillus of black-leg from their local occurrence seem to be distributed from animals infected, though capable of a saprophytic existence outside the body for years. These can no more be attacked during their saprophytic existence than those just mentioned. Doubtless in warm seasons of the year and in the tropics other organisms pathogenic to animals may live and multiply in water or in damp soil where conditions are favorable, just as the cholera organism in India, and occasionally the typhoid bacillus in temperate climates do. (_c_) Most pathogenic organisms, however, when they are thrown off from the bodies of animals, remain quiescent, do not multiply, in fact always tend to die out from lack of all that is implied in a “favorable environment,” food, moisture, temperature, light, etc. Disinfection is sometimes effective in this class of diseases in preventing new cases. II. A.