"There is, however, a wrong impression abroad that because a lowering of the general vitality favors certain diseases, such as tuberculosis, common colds, pneumonia, septic and other infections, it plays a similar rôle in all communicable diseases. Many infections, such as smallpox, measles, yellow fever, tetanus, whooping-cough, typhoid fever, cholera, plague, scarlet fever, and other diseases, have no particular relation whatever to bodily vigor. These diseases often strike down the young and vigorous in the prime of life. The most robust will succumb quickly to tuberculosis if he receives a sufficient dose of the virulent micro-organisms. A good physical condition does not always temper the virulence of the disease; on the contrary, many infections run a particularly severe course in strong and healthy subjects, and, contrariwise, may be mild and benign in the feeble. Physical weakness, therefore, is not necessarily synonymous with increased susceptibility to all infections, although true for some of them. In other words, 'general debility' lowers resistance in a specific, rather than in a general, sense."--(Rosenau: Preventive Medicine and Hygiene, pp. 403 and 404.) CARRIERS Well persons who carry in their bodies pathogenic germs but who themselves have no symptoms of disease are called carriers. Thus typhoid carriers have typhoid bacilli in the intestinal tract, while they themselves show no symptoms of typhoid fever; diphtheria carriers have bacilli of diphtheria in the throat or nose, but have themselves no symptoms of diphtheria, and so on. It has now been proved that many patients harbor bacteria for weeks, months, or even years following an infection, and are dangerous distributors of disease; also, some healthy individuals without a history of illness harbor living bacteria which may infect susceptible persons in the usual ways. Transmission by healthy carriers goes far to explain the occurrence of diseases among persons who have apparently not been exposed. This explanation has greatly clarified the whole problem of the spread of communicable diseases. Carriers, unfortunately, exist in large numbers, and render the ultimate control of disease exceedingly difficult. They can usually be identified by bacteriological tests. To some extent they can be supervised; food handlers at least should be legally obliged to submit to physical examinations, and should be licensed only when proved free from communicable disease.