IMMUNITY 249 



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

 the family may perhaps become weakened by continually 

 breathing such gas, but it is doubtful whether they would 

 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 



