vii INFECTION OF ANIMALS WITH PLAGUE 149 



unnecessary for the successful transmission of plague to 

 man or to rat that B. pestis should have previously been 

 taken up by, not to mention stored within, an intermediary 

 host like the flea. 



The essential point about the transmission of the 

 disease is that the B. pestis per se of a plague case (man 

 or rat) should obtain entrance into a new individual. 

 B. pestis of the laboratory has no particular phases in 

 which only it is specially active ; in no sense, therefore, 

 is it parallel with the malaria parasite in this respect. 

 Active B. pestis has practically but a single phase, namely, 

 that of a bacillus multiplying by fission. This bacillus, 

 whether taken from an active animal source or from an 

 actively growing culture, is always, so long as its normal 

 virulence has not deteriorated, effective on inoculation. 



As was pointed out in a previous chapter as regards 

 a large percentage of rats dying quickly of virulent 

 plague, the blood of the circulation just before death 

 contains the B. pestis sometimes in great numbers, as is 

 shown both by microscopic examination and by culture 

 experiment. It is therefore quite possible that fleas fed 

 on the blood of an animal dying in this state might 

 contain the B. pestis. It was also pointed out that in 

 the virulent or " human " type of plague in the rat the 

 bacilli are more copiously present in the blood of this 

 animal than in the less virulent or "rat" type. Wherefore 

 fleas fed on the blood of a rat affected with the first type 

 of B. pestis would be more likely to contain B. pestis than 

 if fed on the second or " rat " type, and the more com- 

 petent, therefore, by hypothesis, to cutaneously inoculate 

 other rats in proceeding at once to bite them. 



Subject to these reservations, the possibility of trans- 



