IMMUNITY. 73 



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. Vacdnation is an example. Haff- 

 kine'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. 



Immunity Through Inoculations of Small Doses of very 

 Virulent Microorganisms. A graduated resistance to the 

 disease is reached somewhat after nature's method. By succes- 

 sive inoculations with increased doses of the virus an immunity 

 is often reached sufficient to withstand ten times the lethal 

 dose. A poison-habit is thus acquired. 



Increased Virulence is produced as follows : The cultures 

 may be greatly increased in virulence by successive cultivation 

 through animals, and gradually changing from smaller animals 

 to larger, until an amount of the culture that at the outset 

 would not destroy a guinea-pig becomes finally virulent for 

 chickens and dogs. 



Immunity Through Injections of the Sterilized Products of 

 Bacteria. Cultures sterilized by heat or filtration through 

 germ-filters still contain the chemical products of bacteria, the 

 toxins ; and when these are injected in gradually increased doses 

 the same immunity is obtained as with the bacteria themselves. 



Passive Immunity. The blood-serum and tissues generally 

 of animals rendered immune in the ways described above, when 

 injected into susceptible animals render them immune against 

 the same infection. This has been called passive immunity, but 

 there is no strong reason why this term should be used. The 



