234 DISEASES FROM HARMFUL ORGANISMS 



terial from such a cow pressed into an abrasion on his 

 skin. This was the beginning of vaccination. Although 

 bitter hostility was encountered from all sides, the prac- 

 tice almost immediately vindicated itself, and Jenner, dur- 

 ing the later years of his life, was hailed as a great 

 benefactor of his race. The possibility of vaccination 

 rests upon the fact that the organisms of cowpox and 

 virulent smallpox are so nearly identical in their protein 

 composition that the immune bodies which the tissues 

 produce for one serve completely to attack and destroy 

 the other. This close protein relationship of a severe 

 with a mild infection has made it possible to make a 

 very satisfactory beginning toward ridding the civilized 

 world of one of its most fatal and loathsome diseases. 



Defenses of Communities. — As civilization ad- 

 vances and men tend more and more to cluster in com- 

 munities, conditions become more and more favorable 

 for the organisms that produce disease in human beings. 

 These organisms are parasitic within the boches of men 

 and certain kinds of higher animals, so that their chance 

 of existence is greater the more persons are within reach. 

 Furthermore, to maintain the purity of water, milk, 

 and food supplies becomes increasingly difficult as the 

 population becomes denser. The contamination of these 

 substances with organisms that produce disease is for 

 the most part contamination with human excreta. To 

 prevent sewage from getting into drinking water and into 

 the water with which milk-cans are washed seems like 

 a simple undertaking, but in practice it presents a great 

 many difficulties, due cither to indifference on the part 

 of the educated public or to ignorance on the part of 

 others. Communities cannot safely rely wholly upon 

 the intelligent cooperation of all their members, but 

 must organize definite safeguards against contamination 

 of water and food supplies, e.g., water systems, pasteuri- 

 zation of milk, screening of food from flies, etc. 



Contamination of the Air from dried sputum, or 



