Page:Popular Science Monthly Volume 81.djvu/546

540 previous to death, for after this time the bacilli do not occur in the blood. The vitality and virulence of the bacilli are preserved for nearly a week at least and sometimes fully a month; and there is actually an increase in their number during the first few days. Infection from these insects may then occur through their bites, if they contain extremely virulent bacilli, but probably occurs more commonly by the insects being crushed in situ after they have punctured the skin. Plague is confined more generally to the tropics and in recent years has threatened to become epidemic in the United States only in the region bordering San Francisco, Cal. Prompt measures of repression, based on a knowledge of the manner in which the disease spreads, have, however, been very successful and future wide-spread epidemics are not to be expected.

Plague is primarily a disease of rats, and its occurrence as a human disease is rather secondary, so nearly so that it can almost be said "no rats, no plague." On this account the destruction of rats is the first prophylactic measure to be undertaken for the suppression of plague, since this is much more readily accomplished than the destruction of the fleas directly. In parts of California the wild ground squirrel has become infected with plague from rats and presents a menace to the human population, although apparently not so great a one as the rat.

Another disease that has very recently been demonstrated to be insect-borne is typhus fever. This should not be confused with typhoid; it is a very different disease, occurring in the tropics and colder regions alike, and usually associated with dirty, unsanitary surroundings. On this account, it is becoming less prevalent in civilized countries every year, but has at times in the past claimed many victims. During our own civil war, the inmates of army prisons suffered greatly from the ravages of typhus fever, and similar conditions of crowding many people together in filthy surroundings have long been known to be favorable for the development of typhus fever epidemics. We now know through the researches of Ricketts and others that typhus is carried by vermin, the body louse, Pediculus vestimenti, acting as the vector. Thus the etiology of typhus has suddenly been made clear and we are in a position to formulate measures for prophylaxis and quarantine which will prevent the development of the disease in epidemic form. There is much yet to be learned; perhaps other insects also may act as carriers, but there can be no doubt that typhus is almost exclusively insect-borne.

Another disease which has puzzled the medical profession for centuries is a peculiar malady known as pellagra. Pellagra develops very slowly, and the origin of individual cases is correspondingly difficult to trace. It is also usually rather erratic or sporadic in occurrence, but appears to be rapidly increasing in prevalence in many parts of the United States. At one time it was thought that pellagra was contracted