232 BACTERIA, YEASTS, AND MOLDS 



whooping cough, typhoid fever, diphtheria, grippe, tonsilitis^ 

 etc., protects the individual for a time from a second attack. 

 The protection lasts much longer in scJme cases than in 

 others, and whereas the protection against the diseases at 

 the beginning of the above list lasts for years or for life, 

 the protection against those at the end of the list lasts 

 for only a few months or weeks. 



Two important facts in regard to the resistance against 

 disease must be mentioned. The ability of a person to 

 resist an attack of any kind of disease germ is dependent 

 upon two things. 



1 . The vigor of the bacteria. It has been learned by 

 experience that the bacteria reproducing any definite dis- 

 eases are more virulent at some seasons than at others. 

 A very vigorous lot of bacteria will give rise to a more 

 serious attack of the disease, and will be more difficult to 

 drive out than a lot of the same kind of bacteria that 

 have been weakened by some unknown conditions. It 

 is a well-known fact that some epidemics of smallpox, 

 measles, etc., are milder than others ; not simply because 

 fewer people are attacked, but because those who are sick 

 have the disease in a milder form. This difference in the 

 severity of the attack is due in part to a difference in 

 the vigor and activity of the bacteria that make entrance 

 into the body, and is a matter beyond our control. 



2. The vigor of the body itself. A vigorous, healthy, 

 active body has a power of resistance sufficient to drive off 

 most kinds of these invading parasites. If, however, the 

 body is less vigorous, less active, i.e. in a low state of 

 physical health, its resisting power is less and the body has 

 great difficulty in driving off the invaders. This resisting 



