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PLAGUE, EATS AND FLEAS. 259 



proportionate to the severity of the destruction. Tims, if a trap is set, 

 say, in one room which is frequented by rats, they will probably quit that 

 room for a neighbouring room. But if a wholesale destruction is produced , 

 as by placing poison in several places in a house, the rats will quit the 

 house completely. Just so is it with plague. If conditions are such 

 as are unfavourable to the rapid spread of the disease, the infected rats 

 may linger on in a particular house, not being so thoroughly scared by 

 the moderate mortality as to quit the house ; infection may thus smoulder 

 on in a particular house till the conditions become favourable to the 

 extensive spread of the disease. I shall now consider what these fav- 

 ourable conditions are. They are associated with the breeding season 

 of rats, and are due to the increase in the number of susceptible in- 

 dividuals and the multiplication of fleas, the carriers of infection. 



The season at which the greatest number of young rats are present 

 has a twofold influence on the spread of plague. In the first place, the 

 arrival of young members among the community increase the number of 

 individuals susceptible to the disease. 



In the second place, the breeding season is, as a rule, the period of 

 increase of the fleas which are peculiar to the rat. You must be 

 familiar with the fact that kittens and puppies are especially covered 

 with fleas. If you wish to get a particular flea which has a certain 

 bird for its host, your best chance of obtaining that species of flea is to 

 find the bird's nest. So precisely is it with the rats. Rat fleas are 

 most numerous at the time when young rats are most numerous. I 

 fancy I hear somebody say : " Oh ! rats breed all the year round ; they 

 have no seasonal breeding time." This is true and it is not true. Rats 

 do often breed all the year round ; but I am equally certain that there 

 is a season when more young rats are found than at any other season , 

 and this season in Bombay is precisely the plague season. This is a 

 very difficult matter to prove, and I should be very much obliged if 

 any member could devise a practical method by which it could be 

 proved. My inference has been made from observing the number of 

 young rats brought to the Laboratory at certain seasons, and by 

 noting the number of pregnant females which come for post-mortem 

 examination. I am sorry, however, that I have no figures to offer 

 wherewith to support my observation. 



An epizootic of plague among a rat community is very often associated 

 with a sudden and extensive spread of the disease ; in proportion as 



