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 1894; and it has been found that it rarely if ever increases in numbers outside of the body, but rather tends to die off, frequently very rapidly. There is much reason to think that fleas and rats become infected, and are important factors in the spread of the disease, though more evidence 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 introduction 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 observant 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 bacterium, and the larvæ 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