248 BACTERIA, YEASTS, AND MOLDS 



If it is constantly escaping into a house, the members of 

 the family may perhaps become weakened by constantly 

 breathing such gas, and may be more liable to the attack 

 of parasitic diseases. Such persons might perhaps have 

 a tendency to throat troubles ; but there is no evidence 

 in our possession that sewer gas can cause any particular 

 disease. The diseases are caused not by gases but by 

 living bacteria ; and while sewer gas may be deleterious 

 in its weakening action upon individuals breathing it, it 

 can never produce disease. 



Protection following Cure ; Immunity. The recovery 

 from a contagious disease, as a rule, protects the indi- 

 vidual more or less perfectly from a second attack of 

 the same disease. But the amount of protection differs 

 with different diseases. After recovery from some of our 

 contagious diseases, like scarlet fever, a person rarely 

 has a second attack during life. With other diseases a 

 second attack is more likely to follow, but in all cases 

 there is at least a temporary protection following the 

 recovery. In other words, after a person has recovered 

 from a contagious disease he is not, at least for some 

 time, liable to the same disease again. This protection 

 lasts in some cases for many years and perhaps through 

 life (scarlet fever) ; in other cases it may last only a few 

 years (measles?); in some cases perhaps only a few 

 months or weeks (diphtheria); but a temporary protec- 

 tion is always gained. The reason why one is thus pro- 

 tected from a second attack scientists have not yet wholly 

 explained. 



Vaccination. A word must be given in regard to the 

 method of protecting the body against smallpox known 



