IMMUNITY AND SUSCEPTIBILITY. 67 



field-mouse, and white mouse differ very much in their 

 susceptibility to various diseases. 



Acquired immunity is resistance which is the result 

 of accidental circumstances. It may result 



A. By recovery from a mild attack of the disease. 

 Most adults have suffered from rubeola, scarlatina, and 

 varicella in childhood, and in consequence of the attacks 

 are now immune to these diseases i. e. will not become 

 affected again. One attack of yellow fever is always a 

 complete guard against another. Typhoid fever is rarely 

 followed 'by a second attack. 



B. By recovery from an attack of a slightly different 

 disease. Sometimes the immunity is experimentally pro- 

 duced, as when by vaccination we produce the vaccine 

 disease and afterward resist variola. Acquired immunity 

 is a little less complete and not so permanent as natural 

 immunity, for in the latter it is only when the functions 

 of the individual are disturbed or his vitality depressed 

 that the resistance is lost, while in the former time seems 

 to lessen the power of resistance, so that rubeola and 

 scarlatina may return in a few months or years, and for 

 complete protection vaccination may need to be done as 

 often as every seven years. 



C. By the injection of antitoxic substances. At 

 present there is much agitation over the newly-dis- 

 covered antitoxin of diphtheria, the injection of about 

 500 units of which will give complete protection against 

 the disease for a period lasting from a month to six 

 weeks. 



Immunity may be destroyed in numerous ways: 

 (a) By variation from the normal temperature of the 

 animal under observation. Pasteur observed that chick- 

 ens would not take anthrax, and suspected that this 

 immunity might be due to their high body-temperature. 

 After inoculation he plunged the birds into a cold bath, 

 reduced their temperature, and succeeded in destroying 

 their immunity. The experiment was a success, but the 

 reasoning seems to have been faulty, as the sparrow, 



