HEALTH AND DISEASE 413 



and Avater, or they can enter the mouth through the 

 air. Many enter this way, chief of which cause typhoid 

 fever, cholera, and dysentery. 



The respiratory organs afford the best entrance to 

 the body for germs. This is primarily because germs 

 are so easily carried by air. In spite of the many 

 adaptations of the respiratory organs for the removal 

 of dust and germs, the long irregular passageways are 

 good lodging places for them. Tuberculosis, pneu- 

 monia, colds, influenza, tonsilitis, bronchitis, and many 

 other diseases enter the body in this way. 



Fortunately the skin is a very effective covering of 

 the body, and it consequently prevents the entrance of 

 foreign material. If the skin is removed in any way, 

 as in cuts or other injuries, germs can then enter. 

 Those which produce blood poisoning enter the body in 

 this manner. Diseases such as malaria, yellow fever, 

 or bubonic plague are communicated to man by the 

 bites of insects and thus enter the body through the 

 skin. 



Growth of germs. When germs have entered the 

 body, they sometimes find there conditions favorable 

 for their growth; plenty of food and moisture, and a 

 warm temperature. A few will grow any place in 

 the body, but most of them show some choice in their 

 selection of these favorable conditions. Some will 

 grow only in the respiratory tract, others in the ali- 

 mentary canal, and still others only in the blood or 

 skin. 



