244 AGRICULTURAL BACTERIOLOGY 



out life, and is transmitted to the offspring. A striking 

 example of natural immunity is that of Algerian sheep to 

 anthrax, while other varieties are very susceptible to this 

 disease. 



Immunity to a disease may be established in the indi- 

 vidual after birth. It is then called acquired immunity. 

 The lessened susceptibility to certain diseases with increas- 

 ing age is an example, as is noted in measles, chickenpox, 

 and whooping-cough in human beings, and with blackleg 

 in cattle. 



The immunity that is conferred by resisting successfully 

 an attack of infectious disease is another example. In- 

 stances of this type of acquired immunity are noted in 

 smallpox and yellow fever in man, in Texas fever in cattle, 

 and in cholera in hogs. The period of persistence of the 

 acquired immunity is variable, sometimes extending through 

 the remainder of the life of the individual, or again persist- 

 ing but a short time. A successful recovery from some 

 diseases does not seem to convey any immunity against 

 second attack. Acquired immunity may also be produced 

 artificially in a number of ways which may be summarized 

 as follows : 



By inoculating the individual with such a small number 

 of organisms that a fatal attack of the disease will not 

 result. This method is used in the case of Texas fever. 



By inoculating with an attenuated or weakened organism. 

 This is practiced in anthrax, blackleg, rabies, and bubonic 

 plague in man. 



By inoculating with an organism that has been modified 

 by passage through another species of animal. This method 

 is illustrated by vaccine for smallpox. 



By the injection of toxins. This is used in the immuni- 

 zation of animals against the virus of a disease for the pur- 



