IMMUNITY. 107 



variable degree is deleterious to their further growth. 

 It also gives rise to substances which neutralize the 

 poisonous effects of the bacterial products. This im- 

 munity may take place in various ways : 



1. Through recovery from disease naturally con- 

 tracted or from infection artificially produced. This 

 immunity may be slight, as after recovery from 

 erysipelas or pneumonia, marked for a short period of 

 time, as in diphtheria and typhoid fever, or prolonged, 

 as after scarlet fever or syphilis. 



2. By the injection of the bacteria into tissues not 

 well suited to their development, as the injection of 

 typhoid bacilli or cholera spirilla into the subcutaneous 

 tissues. Here a mild local infection follows, with con- 

 siderable resulting immunity. 



3. By the injection of micro-organisms attenuated 

 by heat, chemicals, or other means. In this case a 

 local or general infection of the animal is produced, 

 of moderate severity, as a rule, and the immunity is not 

 as marked and lasting as after recovery from a more 

 serious attack ; but it is, nevertheless, considerable. 

 The inoculation of sheep with the attenuated anthrax 

 bacillus and the use of vaccination in man are examples 

 of this method. 



4. By the injection of the unaltered chemical con- 

 stituents of the dead bodies of bacteria and of the 

 chemical products which they elaborate and discharge 

 into the surrounding culture media during life. Smith 

 and Salmon proved that by repeated injections of the 

 filtered bouillon cultures of the hog-cholera bacillus a 

 considerable immunity may be produced against the in- 

 vasion of this bacillus. Similar results have followed 

 the injections of dead cultures of typhoid and anthrax 



