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 physician Jenner 
observed that persons who had cowpox, a mild form of 
smallpox, were commonly immune to the latter. Reason- 
ing from this he developed the method of vaccination, 
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 sticky substance that exudes from these 
pustules is then collected by sterile instruments and care- 
fully 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 serum or 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 
