END OF FILTH THEORY OF DISEASE. 237 



often attributed to sewer gas. We now know that the only filth to be 

 feared is the secretions of infected persons. 



Bubonic plague has always been classed as a typical filth disease, 

 but here again careful laboratory work has resulted in a vastly clearer 

 knowledge of its causation, though a great deal yet remains to be 

 learned. The bacillus which causes it was discovered by Kitasato in 

 ] 894 ; and it has been found that it rarely if ever increases in numbers 

 outside of the body, but rather tends to die ojff, frequently very rapidly. 

 There is much reason to think that fleas and rats become infected, and 

 iire important factors in the spread of the disease, though more evi- 

 dence on this point is to be desired. In any event it is shown to be a 

 contagious disease, though perhaps not usually directly contagious, and 

 that it does not develop in filth. We might have a perfectly drained 

 city, with modern plumbing, efficient scavenging and the purest of 

 water, yet, if the inhabitants were careless in their habits and opposed 

 isolation, the disease would spread as in an undrained and poorly 

 watered city. It might require rats and fleas to cause an epidemic ; but 

 these animals played no part in the filth theory. 



Tuberculosis was never classed as a filth disease, though the intro- 

 duction of sewers has been held to cause its decrease, it is claimed by 

 draining the soil. It has, however, been proved to be a bacterial disease, 

 but the bacillus will not grow outside of the body and has no relation 

 to filth, except so far as matter expectorated by a consumptive is filth. 



Typhus fever, smallpox, scarlet fever, measles and whooping cough 

 have by some enthusiasts been attributed to filth, but very few observ- 

 ant persons who have studied the distribution of these diseases and 

 followed their outbreaks consider them other than purely contagious. 

 They, of course, never originate in filth or develop in filth, but may 

 spread more among filthy people just because such persons use very 

 little soap and water and allow their faces, hands, belongings and 

 dwellings to become and remain smeared with mucus, saliva, pus and 

 other infectious material. 



Malaria has for centuries been considered to be the product of 

 decaying vegetable matter, but its true relation to such material has 

 only recently been discovered. The mosquito is the bearer of the 

 malarial parasite, which in this case is a protozoan rather than a bac- 

 terium, and the larvge of the particular species of mosquitoes which 

 carry this disease live only in shallow pools where they are protected 

 from their enemies and find an abundance of food. Water which is 

 really filthy is not congenial to them. 



Yellow fever is the one disease which it has been believed could 

 surely be traced to filth. No disease in this country is so dreaded, and 

 its supposed dependence upon filth has made it the last stronghold of 

 the advocates of this theory. It has been held by almost all observers 



