284 HUMAN PHYSIOLOGY CHAP. 



example, those causing smallpox, measles, scarlet fever, 

 and other eruptive diseases common to childhood. 

 These are called contagious diseases. Some germs can 

 live for a long time outside of the body, as in air, water, 

 and foods. These are more dangerous to public health 

 because they can be carried a long distance and attack 

 a whole city or sweep over a continent almost in a 

 day. Diseases of this kind are called infectious. Many 

 diseases, as erysipelas, diphtheria, and consumption, 

 belong to both classes. 



Disease germs enter the body through the mouth and 

 nose and get into the intestines and air passages. The 

 germs found in the air, as those of diphtheria, scarlet 

 fever, and consumption, get into the lungs through the 

 ordinary processes of breathing. Germs contained in 

 water and foods get into the alimentary canal and attack 

 the organs of digestion. Those of typhoid fever, dysen- 

 tery, and cholera belong to this class. These germs 

 cannot travel through the air. The only way to guard 

 against such diseases is to eat good, well-cooked foods 

 and drink water that has been boiled. Some germs gain 

 entrance through wounds or cuts in the skin. The 

 germs of lockjaw always enter in this way, and those of 

 boils, abscesses, and malarial fevers frequently do. In- 

 sects are important agents in conveying germs. Recent 

 investigations show very conclusively that the mos- 

 quito injects the germs of malaria and yellow fever 

 into the blood, and hence we may regard the mosquito 

 as the most important agent in the spread of these 

 diseases. 



