40 



The Capability of Flea Transmission to Interpret the Epi- 

 demiological Facts Regarding Bubonic Plague. 



(1) Poverty, dirt, storage of grain in the bedroom, accumu- 

 lation of refuse, and insanitary conditions generally, have 

 been shown to be effective in so far as they lead to the 

 support of a large rat population in close association with 

 human beings. All these conditions also increase the 

 population of rat fleas, and man's accessibility to them. 



(2) In 75 per cent, of cases the situation of the bubo is 

 such that the bacillus must have obtained entry at some 

 superficial area of the body. This is also consistent with 

 infection by fleas. 



(3) A number of instances have occurred during the last 

 ten years in India in which the introduction into a distant 

 village of the effects of a person dead from plague has been 

 followed, after an interval of about a week, by mortality 

 amongst the rats, and subsequently by an epidemic of plaeue. 

 The same sequence has also taken place after the visit 

 of an individual who had worked or, resided in a house 

 where some of the inmates had suffered from plague, 

 although the stranger himself did not suffer from the 

 disease. These observations are better explained on the 

 assumption that the infection was transported in the bodies 

 of fleas contained in the clothing or upon the person of the 

 stranger than by any other suggestion that has been put 

 forward, for at the earliest opportunity rat fleas would betake 

 themselves to the rats, and could thereby start an epizootic. 

 That such transmission of rat fleas does actually happen has 

 been proved by the Commission for the Investigation of 

 Plague in India, members of which frequently carried away 

 rat fleas upon their persons and clothing when visiting native 

 quarters in the course of their scientific duties. 



(4) Perhaps the most striking feature of epidemics of 

 bubonic plague is the marked seasonal prevalence of the 

 disease amongst rats and amongst men. In places where it 

 is endemic the epizootic followed by the epidemic starts at or 

 about the same time each year, grows, declines, and finally 

 more or less disappears. As one passes away from the 

 Equator the plague season becomes later. In Bombay the 

 height of the epidemic is in March, in Lahore in April, in 

 Jhelum in May, in Rawal Pindi in June, and further north 

 in July, August, and September. The epidemics of London 

 in 1665, and of Marseilles in 1720, reached their maximum 

 in September. The effect of latitude in determining the 

 season when plague flourishes is analogous to that upon the 

 flowering and seeding of a plant, and indicates dependence 

 on some biological factor. Within the Tropics, where 



