228 THE RISE AND FALL OF DISEASE 



Epidemics of })l;i*^iie amongst rats often precede those 

 in man, and dead and sick rats liave always been 

 regarded with the gravest suspicion in countries 

 liable to plague. These observations tlierefore teach 

 us that the rat and the particular Hca M'hicli infests 

 it are the agents which must be regarded as largely 

 responsible for the spread of this disease. And, just 

 as in the case of the mosquito-borne diseases special 

 sanitary measures — the attack, in other words— must be 

 directed against them, so in plague in all anti-plague 

 measures a special war nmst be waged against the rat. 

 Anti-rat measures are enforced ; steps are taken in all 

 epidemics to exterminate them both on ships and 

 in towns. Indeed as a precautionary measure in those 

 places where plague is liable to be introduced, the 

 rats are systematically examined throughout the year, 

 as in Liverpool, to try to detect at the earliest 

 possible stage an acutely or chronically infected rat. 

 The rats on ships coming from ports where plague is 

 known to exist are of course subjected to very special 

 bacteriological scrutiny ; and as a further precaution 

 means are adopted to prevent the rats from such ships 

 getting on shore. By these means, if plague is 

 discovered in the rat the sanitary authority is fore- 

 warned, and is able to take effective steps before the 

 disease has attacked man. Again, just as in the case 

 of yellow^ fever and malaria the essential part of the 

 plan of campaign is to exterminate the anoj^helincs and 

 tlie stcgomyia, so, in the case of plague, the root of 

 prophylaxis is to exterminate the rat, and with it, its 

 own special flea which is the carrier. We see now, 



