270 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol. XVL 



But when even one or two germs are introduced by a needle under 

 the skin, death almost always results. Thus, a guinea-pig has died 

 of plague after receiving one quarter of a c.c. (about 4 drops) of 

 a ten millionfold dilution of a 48-hours' broth culture of plague. 

 In this quantity I was able to show there were not more than one 

 or two germs. If only one or two germs are necessary to infect 

 an animal by injection under the skin, while millions are necessary 

 by any other channel, which is more likely to be the common method of 

 infection ? 



Simond < 10 ) long ago showed that phlyctenules (blisters occasionally 

 found in cases of plague, and which reveal the point of inoculation of 

 the bacillus) often occur in parts little liable to abrasion (such abrasions 

 might permit the plague germs to enter the blood), but which are fre- 

 quently attacked by fleas. If cuts and abrasions were the chief source 

 of infection, phlyctenules would be commonly found on the soles of the 

 feet of those who go about bare-footed, but this is not the case. Phlyc- 

 tenules in this situation are extremely rare, while they are most common 

 just in the situations which are most subject to be attacked by fleas. 

 Again, the boot- wearing people of Australia suffered as frequently from 

 buboes in the groin as the bare-footed natives of Bombay ; — 73 percent, 

 in the one case, 67 in the other ( - 11 \ If the bacillus lived in the soil, 

 it would have been reasonable to expect a larger number of groin 

 buboes in the bare-footed natives than in the booted Australians. 

 Moreover, the common sources of infection with plague are dark, dirty, 

 overcrowded, vermin-infested houses ; also clothing and grain. It is 

 just on these surroundings that fleas may be found. We know, 

 too, that houses and clothing can be indirectly infective through the 

 medium of a healthy uninfected individual. The healthy individual 

 carries with him something, presumably infected fleas, to another indi- 

 vidual who sickens with the disease. 



The seasonal prevalence of plague can to my mind be most easily 

 accounted for by the greater prevalence at some given time of some 

 intermediary host of the plague bacillus after the same manner as the 

 prevalence of malaria is associated with the prevalence, at certain definite 

 seasons, of certain species of mosquitoes of the genus Anopheles. On 

 several occasions fleas have been noted to be specially abundant prior to 

 or during the course of an epidemic of plague. I will only mention 

 2 instances. Thus, for example, Dr. Tid swell's attention was directed 



