In _active_ immunity the organism may be introduced in what is to be considered a natural manner, as when an animal becomes infected, has a disease, without human interference. Or the organism may be purposely introduced to bring about the immunity. For certain purposes the introduction of the products of the organism (toxins) is used to bring about active immunity (preparation of diphtheria and tetanus antitoxin from the horse). The method of producing active immunity by the artificial introduction of the organism is called _vaccination_, and a _vaccine_ must therefore contain the organism. _Vaccines_ for _bacterial_ diseases are frequently called _bacterins_. The use of the blood serum of an immunized animal to confer passive immunity on a second animal is properly called _serum therapy_, and the serum so used is spoken of as an _antiserum_, though the latter word is also used to denote any serum containing any kind of an antibody (Chapters XXVII-XXXI). In a few instances both the organism and an antiserum are used to cause both active and passive immunity (_serum-simultaneous method_ in immunizing against hog cholera). In producing active immunity the organism may be introduced (_a_) _alive and virulent_, but in very small doses, or in combination with an immune serum, as just mentioned for hog cholera. The introduction of the live virulent organism alone is done only experimentally as yet, as it is obviously too dangerous to do in practice, except under the strictest control (introduction of a _single tubercle_ bacillus, followed by gradually increasing numbers--Barber and Webb). More commonly the organisms are introduced (_b_) alive but with their _virulence reduced_ (“attenuated”) in one of several ways: (1) By passing the organism through another animal as is the case with _smallpox vaccine_ derived from a calf or heifer. This method was first introduced by Jenner in 1795 and was the first practical means of preventing disease by _vaccination_. This word was used because material was derived from a cow--Latin _vacca_. (2) By drying the organism, as is done in the preparation of the vaccine for the _Pasteur treatment of rabies_, where the spinal cords of rabbits are dried for varying lengths of time--one to four days, Russian method, one to three days, German method, longer in this country. (It is probable that the passage of the “fixed virus” through the rabbit is as important in this procedure as the drying, since it is doubtful if the “fixed virus” is pathogenic for man.) It would be more correct to speak of this as a _preventive vaccination against rabies_, since the latter is one of the few diseases which is not amenable to _treatment_. The patient always dies if the disease develops. (3) The organism may be attenuated by growing at a temperature above the normal. This is the method used in preparing _anthrax vaccine_ as done by Pasteur originally. (4) Instead of growing at a higher temperature the culture may be heated in such