HISTORIC DEVELOPMENT OF IMMUNOLOGY 39 



the individual by several means : namely, (1) by 

 recovering from a naturally contracted attack 

 of the disease, as occurs after an attack of scar- 

 let fever; (2) by inoculation with nonlethal 

 doses of the virulent organisms; (3) by inocula- 

 tion with attenuated virus, as obtains in vacci- 

 nation against smallpox; (4) by injecting killed 

 bacteria, as in Antityphoid Vaccine; (5) by in- 

 jecting the specific toxins of bacteria, as in the 

 two exceptional instances of the diphtheria and 

 tetanus bacilli which produce such toxins; and 

 (6) by the injection of blood-serum of animals 

 that have recovered from the disease or that 

 have been immunized by any of the above-men- 

 tioned methods, as in the prophylactic inocula- 

 tion against tetanus with antitoxin. Two types 

 of acquired immunity are recognized and re- 

 ferred to as ' ' active ' ' and ' ' passive. ' ' 



Active Immunity An individual acquires an 

 active immunity to certain microorganisms 

 when he himself has survived a natural or modi- 

 fied course of the disease produced by infection 

 with this or that particular organism. In this 

 case, the individual produces his own immunity 

 either because he has had the disease naturally 

 or because it has been intentionally and experi- 



