54o THE POPULAR SCIENCE MONTHLY 



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 ex- 

 tremely 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, how- 

 ever, 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 posi- 

 tion 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 cen- 

 turies 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 



