HOST-PARASITE RELATIONSHIP 49 



Measles was unknown in the Faroe Islands from 1781 to 1846, 

 when it was introduced from Copenhagen. During the next year 

 over 75 per cent of the population developed the disease. 



These two examples indicate that freedom from disease may 

 occur in peoples who are extremely susceptible, merely because 

 they enjoy the freedom from contact. Present quarantine laws 

 have this as one of their objectives. 



The same line of reasoning is applied in hospitals where certain 

 types of infections are isolated to prevent spread. 



REMOVAii OF Primary Host. — The attempt of the French to 

 build the Nicaraguan Canal failed because of malaria and yellow 

 fever. The U. S. Government was able to maintain a healthy per- 

 sonnel of labor in the same environment by eliminating mosquitoes, 

 the primary hosts for the infectious agents concerned, rather than 

 by the use of immune individuals in the construction work. Such 

 control may depend upon two possible factors: one is personal 

 prophylaxis against contact with the primary host and the second 

 is the limitation of the habitat of the same. It is obvious that in 

 areas where the primary host cannot survive, a break in the chain 

 of infection will protect the human population. The United States 

 is practically free from bubonic plague. The Public Health Serv- 

 ice has combated this disease by measures designed to eradicate the 

 natural host or rodent rather than the insect carrier. During the 

 World War delousing measures were employed extensively in order 

 to prevent the spread of typhus fever and trench fever. 



Sanitary Measures. — Mills-Reincke Phenomenon. It has 

 been observed that when a large city water supply which has l)een 

 consistently polluted with sewage is purified by the introduction 

 of filtration and treatment methods, the morbidity and mortality 

 of diseases other than gastrointestinal in origin are noticeably re- 

 duced. This is another example of protection by removal of causes 

 l)y sanitary measures. 



It is quite generally conceded that our low morbidity rate for 

 typhoid fever today is due largely to sanitary measures relative to 

 water supplies, milk supplies, oyster industry, and also to food 

 handlers, since only a small percentage of our population has 

 typhoid fever or has been immunized. The corollary to this is the 

 relatively high incidence of tyjilioid fever among children who are 

 essentially the milk drinkers of the population. 



