PARASITES 427 



of from 42 to 43C., or 108% to 109%F., they gradually 

 lose their VIRULENCE, or disease-producing power, but do /lot 

 lose the power to stimulate the animal to build up its re- 

 sistance to the disease. 



492. Preparation and Application of Anthrax Vaccine. In 

 the ordinary practice of preparing the vaccine, bacteria whose 

 virulence has been tested by inoculation experiments on 

 rabbits or guinea-pigs are grown at the temperature of from 

 42 to 43C. for varying lengths of time. Whenever it is found 

 that the culture has been so weakened that it will just kill 

 white mice but not quite kill guinea-pigs, about Y c.c. of the 

 culture is injected into cattle and half this amount into sheep. 

 About twelve days later, a similar dose of a culture that will 

 just kill guinea-pigs but not quite kill rabbits is injected. 

 After this, virulent cultures may be injected with impunity, 

 and the animal so treated will not contract the disease in any 

 natural way. 



493. Nature of Immunity. When an organism is free from 

 liability of attack by a given disease, it is said to be IMMUNE 

 or to possess IMMUNITY from the disease. All organisms are 

 naturally immune to certain diseases that affect other organ- 

 isms. Thus man is not at all susceptible to chicken cholera 

 or to a good many other diseases that affect domestic animals 

 and, on the other hand, the domestic animals are not affected 

 by very many of the diseases that affect man. We call this 

 sort of immunity, NATURAL IMMUNITY. But you are familiar 

 with the fact that when you have once had an attack of 

 measles or whooping cough you are not liable to have these 

 diseases again, even though you be repeatedly exposed to them. 

 You are thus rendered immune to these and other diseases by 

 having had them. This kind of immunity, we call ACQUIRED 

 IMMUNITY. You will note that the method of vaccinating an 

 animal with an attenuated culture of the organism that causes 

 a certain disease gives the animal an acquired immunity against 

 the disease without the necessity of the animal's having the 

 disease. 



