EPIDEMIOLOGY.) . Subject to the general conditions mentioned above the impor tance of endemic disease to a community can best be illustrated by a few specific examples.
Diphtheria.—It is certain that in the throats of some appar ently normal individuals bacilli are present from time to time, or constantly, which are indistinguishable (except perhaps in some cases or occasions on bacteriological examination) from B. diph theria. That the individual does not suffer from the clinical disease known as diphtheria depends upon the fact that his active immun ity is kept up by reason of his repeated re-inoculation with small doses of the bacteria in his own throat and the toxin they produce. So far the condition concerns the individual alone, but suppose he be a child attending school and that the weather be cold and wet so that the children are confined to close quarters and many of them are afflicted with slight "colds and coughs." Under these conditions the chance of some of these diphtheria bacilli exhaled or expectorated by the "carrier" finding lodgement in the throats of other children is enormously increased. Moreover it is probable under the given conditions that the resistance of these other children to infectious disease will be below normal, and that quite apart from their specific reaction towards B. diphtheria as can be determined by the Schick test. The conditions are eminently favourable for an outbreak (local epidemic) of diphtheria.
Typhoid and Paratyphoid Fevers.—In the case of these diseases the "carrier" side of the problem is similar to that obtain ing in diphtheria, but in addition the cultural characteristics of the offending micro-organism play a large part. Not only are the bacteria expelled in enormously larger numbers, but also they contaminate sewage and may, by leakage, contaminate the water supply. Moreover they multiply under saprophytic conditions to a far greater degree than B. diphtheria, for example. Hence the differences between various endemic diseases, though perhaps relatively unimportant so far as concerns the individual, may be of great importance to the community.
Plague.—In this bacterial disease the endemic condition really concerns rats and the fleas they harbour. Actual cases of human plague may be wanting for a time in a community liable to out breaks and it is only when mortality among the rats causes their parasitic fleas to leave the rodents and attack man that human cases of bubonic or pneumonic plague arise. Hence endemicity in the case of plague is not quite the same phenomenon as in the case of the other diseases we have considered.
Typhus Fever; Malaria.—Mutatis mutandis, endemic con ditions in these diseases resemble those in bubonic plague. That is to say, the environmental conditions influencing the life-history of the body-louse and the anopheline mosquito, respectively, play dominant parts in the occurrence and dissemination of the two diseases. Nevertheless these factors could not lead to epidemics were it not for the existence of sporadic human cases in the com munity. And were it not for density of population, need of warmth, conditions of poverty, facility of transport in the case of typhus and for warmth and moisture favouring the multiplication of mosquitoes in the case of malaria, sporadic cases of these two diseases might occur in the community, or even the diseases might be endemic but epidemics would not arise.
Filter-passing Viruses.—In the case of some diseases believed to be due to the action of filter-passing viruses (q.v.), e.g., vari cella, dengue, distemper, foot-and-mouth disease, some varieties of herpes, there is evidence from the more or less frequent occur rence of epidemics and the outbreak of disease in newcomers into the district that on occasions the respective diseases may be endemic. The conditions underlying these outbreaks of disease are even less well understood than those described, mainly because the nature of the aetiological factor is unknown.