Immunity may be considered with reference to a single individual or to entire divisions of the organic world, with all grades between. Thus plants are immune to the diseases affecting animals; invertebrates to vertebrate diseases; cold-blooded animals to those of warm blood; man is immune to most of the diseases affecting other mammals; the rat to anthrax, which affects other rodents and most mammals; the well-known race of Algerian sheep is likewise immune to anthrax while other sheep are susceptible; the negro appears more resistant to yellow fever than the white; some few individuals in a herd of hogs always escape an epizoötic of hog cholera, etc. Immunity within a given species is modified by a number of factors--age, state of nutrition, extremes of heat or cold, fatigue, excesses of any kind, in fact, anything which tends to lower the “normal healthy tone” of an animal also tends to lower its resistance. Children appear more susceptible to scarlet fever, measles, whooping-cough, etc., than adults; young cattle more frequently have black-leg than older ones (these apparently greater susceptibilities may be due in part to the fact that most of the older individuals have had the diseases when young and are immune for this reason). Animals weakened by hunger or thirst succumb to infection more readily. Frogs and chickens are immune to tetanus, but if the former be put in water and warmed up to and kept, at about 37°, and the latter be chilled for several hours in ice-water, then each may be infected. Pneumonia frequently follows exposure to cold. The immune rat may be given anthrax if first he is made to run in a “squirrel cage” until exhausted. Alcoholics are far less resistant to infection than temperate individuals. “Worry,” mental anguish, tend to predispose to infection. The following outlines summarize the different, classifications of immunity so far as mammals are concerned for the purposes of discussion. Immunity. { {1. Inherited through { { the germ cell or cells. { { {(_a_) By having the {A. Congenital {2. Acquired { disease _in utero_. I. Natural { { _in utero_. {(_b_) By absorption { { { of immune { { substances {B. Acquired by { from the mother. { having the disease. II. Artificial--acquired through human agency by: 1. Introduction of the organism or its products. 2. Introduction of the blood serum of an immune animal. Immunity.