78 ESSENTIALS OF BACTERIOLOGY 



CHAPTER XIV 



IMMUNITY 



Immunity, as distinguished from susceptibility, is merely a 

 relative term, as no animal is absolutely immune under all condi- 

 tions. It is merely less susceptible, and some animals are by 

 nature or can by artificial means be rendered so slightly sus- 

 ceptible that to all practical purposes they are immune that is, 

 capable of resisting an attack of the particular disease against 

 which they are said to be immune. 



Natural Immunity. The goat and dog are considered 

 naturally immune to tuberculosis. Algerian sheep are resistant 

 to anthrax; other varieties are susceptible. 



The field-mouse is susceptible to glanders; the white mouse 

 is ordinarily immune. House mice are susceptible to mouse 

 septicemia; field-mice are immune. 



Acquired Immunity. Immunity can be acquired in many 

 ways. Active and passive immunity are varieties. 



Active immunity can be acquired from an attack of the disease; 

 such infectious diseases as measles, scarlatina, and whooping- 

 cough usually confer immunity from future attacks. Some 

 diseases render the individual immune for only a short period. 



Immunity from Inoculation with Attenuated or Weak- 

 ened Cultures of Bacteria. Vaccination is an example. 

 Haffkine's cholera vaccines and Pasteur's vaccines of anthrax 

 and chicken cholera are likewise examples of this method. 



Attenuation is produced as follows: Successive cultivation in 

 artificial media destroys the virulence of bacteria. Old cultures 

 are less virulent than fresh ones. Virulence is lessened by 

 passing the cultures through animals that are less susceptible 

 or entirely immune. The cautious use of chemicals and sun- 

 light lessens virulence. Heat is an effective agent. An anthrax 

 culture exposed to a temperature of 42.6 C. for twenty days 

 will prove destructive only to animals no larger than mice. 

 Prolonged exposure to oxygen weakens the germs. 



