THE GEOGRAPHY OF DISEASE, AND THE FACTORS WHICH DETERMINE IT

Resumo

The study which forms the subject of this volume differs in many respects from a study of the geographical distribution of more concrete objects, such as certain groups of animals, plants, or minerals. Though constantly spoken of as if it were a material, tangible entity, disease is, in fact, no such thing. It is only a morbid phenomenon, or rather a group of morbid processes, in the tissues of a particular animal organism. In the language of logic, it is not even a phenomenon, but an epiphenomenon. It is only in that class of diseases known or believed to be of Parasitic origin that there exists, in addition to the group of intangible signs and symptoms which in ordinary language constitute the disease, a tangible, palpable something, the distribution of which over the earth's surface may be justly compared with the distribution of mammals or insects, herbs or trees. But, on the one hand, it has not yet been shown that all, or nearly all, diseases are of this character; and, on the other hand, even where the parasitic origin of a human disease has been proved, it is well to bear in mind that the parasite and the disease are not one and the same thing, nor is their geographical distribution always or of necessity identical. There is good reason to believe, for example, that the parasites which are the known or suspected causes of such diseases as cholera, blackwater fever, malaria, guinea-worm disease, hydatids, and perhaps enteric and other fevers, may and do exist for long periods together outside the human body, and that there are uninhabited or sparsely inhabited tracts of the earth's surface where these parasites remain in the soil, or in water, or in the bodies of the lower animals, and where the human disease associated with them is only set up when man visits those tracts. In other words, the area of distribution of some disease parasites may be wider than the area of distribution of the human disease caused by them. In these instances the disease has a wider distribution potentially than actually. In others the reverse may be the case, and the area of distribution of the disease may at any given moment be actually wider than that of the parasite which gave rise to it. This, however, is exceptional, and can only occur in the case of certain affections of long duration, such, for example, as elephantiasis arabum, where the symptoms of the disease remain long after the filaria or other parasite which first caused them has disappeared from the tissues.