314 STRUCTURE AND LIFE HISTORIES 



an antitoxin which counteracts the influence of the toxin 

 (Cf. p. 310). The production of these antitoxins may 

 finally completely nullify the effect of the toxin, and then 

 the patient ''gets well." The presence of the antitoxin, 

 thus produced, explains why one who has recently 

 recovered from a contagious disease, like measles, or 

 mumps, or whooping cough, is more or less immune for 

 a longer or shorter period. In 1796 the English physician 

 Jenner observed that persons who had cowpox, a mild 

 form of smallpox, were commonly immune to the latter. 

 Reasoning from this he developed the method of vaccina- 

 tion. By this method the cowpox is first given to a calf 

 or a heifer, or sometimes to an adult cow. At the end 

 of five to seven days pustules occur on the infected surface 

 of the animal. A watery substance within these pustules 

 is then collected by sterile instruments and carefully 

 tested to make sure that it does not contain any germs 

 of tuberculosis or other disease. This substance is the 

 vaccine, and in vaccination a small portion of it is applied 

 to a scratched or slightly lacerated area on a person's 

 arm. A mild form of the disease results, causing the 

 formation of an antitoxin in the person's blood, and 

 thus rendering him actively immune. The word vaccina- 

 tion is derived from the Latin word vacca (a cow) , in allu- 

 sion to the method of obtaining the vaccine. It has been 

 calculated that, in large armies, fully as many lives have 

 been saved from disease by vaccination against typhoid, 

 cholera, and other diseases as are lost in battle. 



6. Serum-therapy. The treatment of germ diseases 

 by serum-therapy consists in injecting into the blood of 

 the patient an antitoxin, specific for the disease to be 

 treated. The antitoxin is contained in the blood-serum 



