not well satisfied in countries where the heat renders necessary the admission of large volumes of fresh air into the dwellings of the inhabitants, nor in colder countries, where the art of building has so little advanced that large volumes of air enter the dwellings in spite of the mistaken endeavours of the inhabitants. Again, as regards scarlatina, undiscovered causes in the environment are unfavourable to the existence of the virus in Arabia; the disease is therefore almost unknown in that country, though its inhabitants are not immune when they travel abroad.
In the third class are included all zymotic diseases of which the microbes are able to subsist for an indefinite time outside the living body on non-living organic matter, the microbes of which, in fact, are essentially saphrophitic, and have their normal habitat outside the living body, but are capable of existence, should occasion serve, within it. Examples of such are the malarial fevers. Unlike the diseases of the first two classes, the prevalence of their germs in any territory is quite independent of the human population, except in so far as it affects the outside conditions through drainage, deforestry, &c., and therefore the inhabitants of the most sparsely peopled lands may be afflicted by them.
These three classes of diseases are not sharply defined from one another, but shade, the first into the second, and the second into the third. From diseases, the germs of which are practically incapable of existence outside the living body, to diseases, the germs of which have their normal habitat external to it, every degree of ability to exist outside the body may be observed. The microbes of some diseases,—e.g. syphilis,—practically speaking, cannot exist outside the living body; the microbes of others—e.g. tuberculosis—can exist, but cannot multiply outside it; the microbes of others—e.g.
