CONTROL OF DISEASE 343 



white corpuscles. Later, when typhoid germs get into the body, they are 

 dissolved by the cytolysin already present. 



Other specific cytolysins have been produced and will be of value for 

 diagnosis, for identification of materials, and for the cure or prevention 

 of disease. 



261. Vaccination. The blood meets the invasion of foreign 

 bodies or foreign substances in several different ways. These re- 

 actions depend upon the vital properties of the cells of the body, 

 and especially of the white corpuscles. By understanding these 

 blood reactions we can increase the immunity of the body with 

 respect to certain diseases, and even develop artificial immunity. 



In the case of diphtheria and of some other diseases the use 

 of antitoxin establishes what we may call a passive immunity, 

 since here the blood of the patient does nothing actively; it 

 simply makes use of the previous activity of some other ani- 

 mal's blood. Such immunity usually lasts but a limited time. 



An active immunity is one acquired by stimulating the cells 

 to produce their own counteracting substances. In typhoid 

 fever, for example, the injection of dead bacilli stimulates the 

 body to produce a typhoid cytolysin. The process of inducing 

 an active immunity is called vaccination. Successful methods 

 of vaccination are now in use against several diseases besides 

 typhoid and smallpox. That vaccination is a genuine means of 

 control is demonstrated by comparing death rates from small- 

 pox in different communities (Fig. 162), or by comparing 

 typhoid death rates in the army, which used vaccination, with 

 those in the civil population (Fig. 163). 



The term vaccination comes from the Latin vacca (cow), and was 

 first applied to the practice of infecting people with pus from cowpox, 

 about the end of the eighteenth century. Observers had found that 

 those who had a mild attack of pox, by infection from a cow. were later 

 immune to smallpox. This disease w^as so common at one time that 

 hardly one person out of ten escaped it. That cowpox is a variety of 

 smallpox, and that the germs of cowpox do actually make human beings 

 immune to smallpox, was established by Edward Jenner through careful 

 experiments. 



