92 ORIENTAL PLAGUE chap. 



that, so far from the rat being the chief source of transmission of 

 plague to man, it is of minor importance among the agencies, known 

 or unknown, by which such transmission takes place. 



Review of all the facts set out in the foregoing pages leads, in 

 my judgment, to the conclusion that the part played by the rat in 

 transmission of plague to man, although real, falls far short of 

 the importance which has generally been attributed to it. That it 

 should be so over-estimated is a matter of serious practical moment, 

 since such a view may lead to comparative neglect of other risks of 

 infection and to the imposition of measures, in relation with rats, 

 of unwarrantable stringency. 



That the rat should have attained this position in the minds of 

 many, as an agency in the propagation of plague, is easily compre- 

 hensible. The tracing of the causation of disease is ever present 

 with the medical investigator ; the rat suffers readily from plague, 

 and is capable of transmitting this disease to man; and wherever 

 man is, there the rat abounds. When, therefore, other cause of 

 plague in man is not readily apparent, the tendency is to blame the 

 rat, always there and notoriously liable to this malady. But we 

 know, from the study of other infectious diseases, that, in this class 

 of malady generally, infection is wont to spread in ways beyond our 

 comprehension ; in this respect plague has taught us nothing new. 

 It is only by impartial and thorough consideration of all the circum- 

 stances that the degree of importance of a particular agency of 

 infection can be correctly estimated. My view as regards those who 

 believe the rat to be the chief agency whereby plague is transmitted 

 to man is that they have paid too much attention to individual 

 instances, of an impressive character, in which the rat has given 

 plague to man, and, thus unduly influenced, have failed to weigh 

 the whole evidence, negative as well as positive." 



There is one further point not included in Dr. 

 Thomson's pages ; it is in my opinion of great importance 

 in estimating the amount of danger of infection of man 

 with plague from the rat. I have shown (Medical Officer's 

 Keport, 1902-1903 and 1903-1904) that B. pestis bred 

 in the rat is of decidedly lesser virulence than that bred 

 in the human subject ; moreover, the former is liable, 

 outside the animal body, to a much greater extent to 



