206 BACTERIA, YEASTS, AND MOLDS 



If, however, they overcome these resisting forces and gain 

 a foothold, they then begin to develop, so that in the 

 course of a few days they become quite numerous. As 

 they grow they produce their toxins, and these, devel- 

 oped at first in small quantity, are absorbed by the body 

 and give rise to the first slight symptoms characteristic 

 of the particular disease. But the bacteria continue to 

 multiply and produce their poisons in greater and greater 

 abundance. As a natural consequence the body becomes 

 more and more influenced by them, the symptoms of the 

 disease become more and more violent, the person becomes 

 more and more ill. This continues until death occurs or 

 a crisis is reached. After the crisis the bacteria begin to 

 disappear, and are finally driven from the body, while the 

 poisons they produced become less capable of causing 

 injury and are eventually excreted. The person may then 

 recover entirely from the attack. 



RESISTANCE AGAINST DISEASE 



In most cases the body in driving off the bacteria 

 acquires the power of guarding itself from a second attack 

 of the same species, and the individual, for a time at least, 

 is not liable to a second attack of the same disease. The 

 whole explanation of how the body protects itself, drives 

 off the invading bacteria, counteracts their toxins, and 

 retains this power of protection in the future, is one of 

 the interesting problems upon which bacteriologists are 

 still studying. We cannot here enter into the subject, but 

 it is well to remember that a recovery from common con- 

 tagious diseases, like smallpox, scarlet fever, measles, mumps, 



