114 IMMUNITY AND INFECTION 



excreted during life, or products arising from their disintegration. 

 Immunity to smallpox following vaccination' and immunity to typhoid 

 fever following the injection of killed cultures of typhoid bacilli are 

 familiar examples of this type of immunity. 



There is usually a period of increased susceptibility to infection 

 immediately after the introduction of the virus or its products, in 

 artificially acquired immunity. This period of susceptibility is fol- 

 lowed by an increase in resistance to the virus. If the process of 

 immunization is repeated several times, the initial level of resistance 

 to infection may be raised very materially. Thus, prophylactic vac- 

 cination with killed typhoid bacilli (anti-typhoid vaccination) increases 

 the resistance of the recipient of the vaccine to typhoid infection to 

 such a degree that his chances of acquiring the disease are greatly 

 lessened. It is also probable that in the event of infection of the 

 protected individual with the typhoid bacillus, both the- duration and 

 severity of the attack will be diminished. 



2. Passive Immunity. (a) Antibody Immunity. Introduction into 

 the host of specific products of immunity (antibodies) as diphtheria 

 antitoxin. 



(6) Chemotherapy. The use of chemicals for preventing or modifying 

 infection. 



Passive immunity is induced by the injection of antibodies into 

 the host, which have been developed in another animal. The recipient 

 of these antibodies is protected only so long as they remain in the 

 body. The immunity, however, is effective almost immediately after 

 injection; there is no latent period. 



3. Mixed, Active and Passive Immunity. Mixed artificially acquired 

 immunity is induced by the simultaneous injection of specific anti- 

 bodies and the weakened or attenuated virus; resistance to infection 

 is usually increased at once (passive immunity), while at the same 

 time the host begins to react to the virus and to produce antibodies 

 thereto (artificially acquired immunity). 



The factors which predispose the host to or protect him from inva- 

 sion by microorganisms are usually varied and complex. Relatively 

 simple explanations of the mechanism involved suffice to account for 

 the phenomenon in specific instances, however. For example, frogs 

 and hens are not naturally susceptible to infection with the anthrax 

 bacillus, whose optimum temperature of growth is 37 C., yet infection 

 could take place if the body temperature of either animal were brought 

 to this level, as Pasteur showed nearly two decades ago. A change 



