The characteristic of a contagious disease is its specificity; that is, the disease transmitted is always the same in its essential characteristics. It does not, however, follow that all cases of the disease are equally liable or have the same power to transmit it; in other words, the degree of virulence of the contagiousness is not an essential characteristic. That the same disease sometimes spreads rapidly and is very fatal, and at other times seems hardly to have any contagious properties and is very mild, has long been noticed, and has been attributed to an unknown something called the medical constitution of the place--the constitution médicale of French writers. The true cause is probably very complex, but in some cases, at all events, it seems to be due to difference in the contagion itself. If we suppose this contagion to be a minute organism, it is easy to form a theory as to the cause of these differences, but there is much careful experimental work to be done before we shall have positive knowledge on this point. The results obtained by Pasteur in attenuating the virus of chicken cholera and splenic fever indicate one line which these experiments will take, and the researches of Koch point out another. The diseases which spread by contagion until they form epidemics are those which have from the earliest times attracted the most general attention, and which have given rise to organized efforts for prevention--_i.e._ to public hygiene. They are also the diseases which have given rise to the most bitter controversies among medical men as to the means of their propagation and the best methods of prevention. Plague, cholera, yellow fever, and typhus are those with regard to which this difference of opinion has chiefly occurred--one party considering their chief cause to be contagion, or specific germs derived directly or indirectly from the bodies of the sick; the second party declaring that they are due to filth plus an unknown something, which is variously termed epidemic constitution, pandemic wave, Providence, or _x_. The great majority of opinions at present is in favor of the view that they are all contagious, but not all, or always, contagious from person to person--that they spread from infected localities, which localities receive their infection from cases of the disease. The best means of dealing with them under ordinary circumstances are now tolerably well understood, and where these means can be commanded--as, for instance, among troops in time of peace--epidemics of these diseases can be stopped with great precision and promptness by isolation and disinfection. {201} By "isolation" is meant not only the separation of the sick from the well, but the isolation of the infected locality or water-supply until it has been rendered harmless.