includes ship-fever, jail-fever, camp-fever and Rocky Mountain spotted 

 fever. The Rocky Mountain fever is transmitted by a tick (see page 616). 

 Since the flea is comparatively rare in the United States, ship-fever, jail-fever 

 and camp-fever have not become epidemic here. 



Are All Diseases Caused by Parasites? 



The Fight against Specific Diseases We have succeeded remarkably 

 well in preventi?7g communicable diseases. We have not exterminated all 

 specific or communicable diseases, of course, and probably never shall. But 

 we have completely exterminated some of them in some areas (see illustra- 

 tion opposite). 



In the early part of the century, rates for various communicable diseases 

 fluctuated irregularly. Later there was a steady decline in the incidence of 

 many of these diseases, and especially in the mortality which they caused. 

 In the chart on the opposite page, the figures on the left of each graph in- 

 dicate the number of deaths per year in the case of each disease, for 100,000 

 persons. For the general death rate, however, the figures are per thousand 

 of population. 



It is always a particular person who is well — or sick. Yet most individuals, 

 whether as patients or as potential victims of infection, have done very little 

 to reduce or eliminate communicable diseases. We go about our affairs pro- 

 tected by experts and specialists of whose existence most of us do not even 

 know. Increasingly, however, each of us must co-operate if the optimum re- 

 sults are to be attained. We notify health officers of the existence of com- 

 municable diseases. We remain at home when we suspect an infectious dis- 

 ease. We avoid acts that may endanger others, such as spitting, disposing 

 carelessly of refuse, smoke, vapors, dust, and so on. We have to accept in- 

 spection of our premises or persons, vaccination and other immunizations, and 

 quarantine regulations. We do many things that we would not do sponta- 

 neously or would not do wilhngly if left to ourselves. We avoid doing what we 

 should otherwise want to do. To get the benefits of science we have to accept 

 numerous regulations, restraints of our "personal liberties". 



Nonspecific Diseases The revolutionary results of the germ theory 

 made it reasonable to suspect parasites in every disease. We have learned, 

 however, that the metabolism may be disturbed by a variety of "causes" 

 other than infections. Specific deficiencies — or excesses — in diet may modify 

 growth or development, and so result in distinct diseases. Simple goiter, for 

 example, has been traced to a shortage of iodine. A faulty balance of calcium 

 and phosphorus seems to influence unfavorably the development of bones and 

 teeth. An excess of selenium in the soil brings about a sick condition in cattle, 

 and probably in human beings too by way of the plants they eat. Pellagra, 



626 



