276 BACTERIA, YEASTS, AND MOLDS 



Where sewage leaks, flies are sure to gather, and disease 

 may be carried by them even to people outside the imme- 

 diate household. The evils from sewer gas have, however, 

 been overrated. Sewer gas itself is not capable of producing 

 any specific disease. At one time it was thought that sewer 

 gas was a great menace and likely to produce various diseases 

 in those breathing it. Further experience has shown that 

 this is not true. The gas is unpleasant but not in itself harm- 

 ful. The diseases are caused not by gases but by living 

 bacteria, and sewer gas 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 thrdugh 

 life ; in other cases it may last only a few years ; in some 

 cases perhaps only a few months or weeks ; but a temporary 

 protection is always gained. The reason why one is thus 

 protected from a second attack scientists have not yet 

 wholly explained, but its practical importance is beyond 

 question. 



Vaccination. A word must be given in regard to the 

 method of protecting the body against smallpox known 



