I. Active--due to the introduction of the organism or due to the introduction of the products of the organism. A. Naturally by having the disease. B. Artificially. 1. By introducing the organism: {1. Passage through another animal. {2. Drying. (_a_) Alive and virulent. {3. Growing at a higher temperature. (_b_) Alive and virulence {4. Heating the cultures. reduced by {5. Treating with chemicals. (_c_) Dead. {6. Sensitizing. {7. Cultivation on artificial media. 2. By introducing the products of the organism. II. Passive--due to the introduction of the blood serum of an actively immunized animal. Immunity present in an animal and not due to human interference is to be regarded as _natural_ immunity, while if brought about by man’s effort it is considered _artificial_. Those cases of natural immunity mentioned above which are common to divisions, classes, orders, families, species or races of organisms and to those few individuals where no special cause is discoverable, must be regarded as instances of true _inheritance_ through the germ cell as other characteristics are. All other kinds of immunity are _acquired_. Occasionally young are born with every evidence that they have had a disease _in utero_ and are thereafter as immune as though the attack had occurred after birth (“small-pox babies,” “hog-cholera pigs”). Experiment has shown that immune substances may pass from the blood of the mother to the fetus _in utero_ and the young be immune for a time after birth (tetanus). This is of no practical value as yet. It is a familiar fact that with most infectious diseases recovery from one attack confers a more or less lasting immunity, though there are marked exceptions. =Active Immunity.=--By active immunity is meant that which is due to the actual introduction of the organism, or in some cases of its products. The term active is used because the body cells of the animal immunized perform the real work of bringing about the immunity as will be discussed later. In _passive_ immunity the blood serum of an actively immunized animal is introduced into a second animal, which thereupon becomes immune, though its cells are not concerned in the process. The animal is _passive_, just as a test-tube, in which a reaction takes place, plays no other part than that of a passive container for the reagents.