286 HUMAN PHYSIOLOGY CHAP. 



the blood are active in destroying certain bacteria, that 

 the plasma of the blood and lymph can kill bacteria, 

 and that the serum of the blood may contain antitoxins 

 which poison the bacteria. A person may become 

 immune to certain diseases by vaccination, protective 

 inoculation, and other methods in use at present. In 

 some of the infectious diseases the body becomes so 

 changed that a second attack seldom occurs, as in scar- 

 let fever, yellow fever, smallpox, measles, and mumps ; 

 while in others the immunity lasts only for a short time, 

 as in diphtheria. 



If the body cannot successfully resist the growth of 

 the germs or the poisons they produce, it must yield to 

 the attack of the bacteria. Most of the diseases due to 

 bacteria are commonly known as fevers, which must 

 run a certain course. Of course, fever means simply 

 a condition of the body in which there is a rise in tem- 

 perature, and may be due to many other causes. The 

 course of those due to germs may be described as 

 follows : 



1. Infection, or the entrance of the germs into the 

 body. 



2. Incubation, or the period of growth in the body 

 without any bad signs of their presence. 



3. Onset, or the time the symptoms of disease begin 

 to show themselves. 



4. Height, or the time the fever reaches its highest 

 point. 



5. Decline, or the time the temperature falls to the 

 normal. 



