THE CONTROL OF HEALTH 381 



Chicken-pox 20 days 



Whooping cough 21 



Cholera 10 " 



Typhoid fever ........ 14 



Mumps 24 " 



Infantile Paralysis 28 " 



Immunity. In a previous chapter (page 72) we have 

 defined immunity as a condition of the body such that it is 

 not susceptible to a communicable disease even when inti- 

 mately exposed to it. For example, the same mosquito 

 (genus Anopheles, and carrying malarial germs) biting a man 

 and a bird, conveys malarial fever to the man but not to the 

 bird. Another genus of mosquito (Culex) biting both, causes 

 malaria in the bird but not in man. Each organism is naturally 

 immune to the bacteria which cause a disease in the other. 



Immunity cannot in any way be secured against some 

 maladies, e. g. tuberculosis. Immunity is being secured 

 against an increasing number of diseases by artificial and 

 harmless means. The discoveries made by Louis Pasteur 

 marked what may justly be considered the most important 

 epoch in the history of medicine, and what may almost be 

 called the physical salvation of mankind. 



To understand how immunity may be brought about 

 requires attention to the relation which exists between the 

 invading bacteria and the person invaded. 



When disease bacteria from any source obtain entrance to 

 the human body, they soon become vigorous and multiply 

 very rapidly because of the warm temperature and of the 

 nutrition they so easily absorb from the body fluids. Under 

 favorable circumstances a single bacterium of certain kinds 

 may give rise to as many as 17,000,000 descendants in twenty- 

 four hours. Passing from the respiratory or digestive organs 

 into the blood or lymph, they give from their bodies substances 

 called toxins, which are often very poisonous, and cause the 

 discomfort and symptoms of th disease, i The cells of the 



