146 



Moreover, the gross and microscopic lesions in the lymph nodes are 

 practically the same, and the B. pestis recovered in both fulfills the 

 postulates of Koch. 



Nothing was definitely known, however, of the mode of transmis- 

 sion of the disease from rat to rat or from rat to man until the com- 

 pletion of the experimental work of the Indian Plague Commission. 

 Simond, Ogata, Thompson, and Koch each expressed the belief that 

 the infection was transferred by the rat flea. Nuttall (1897) and 

 Simond (1898) demonstrated the presence of B. pestis in the bodies 

 of bugs (Cimex) and fleas which had been taken from plague-sick 

 rats, and the latter observer, in the same year, succeeded in trans- 

 mitting the disease from rat to rat without contact. 



The work of the Indian Plague Commission was undertaken (1905) 

 with a view to establishing the exact relationship between epizootics 

 among rats and epidemics among men, and included both field and 

 laboratory observations. The experiments of Gauthier and Ray- 

 baud (1903) and of Simond were repeated on a larger scale and greatly 

 improved in that all rats and fleas used were first identified as to 

 species. The findings of the commission may be briefly summarized 

 as follows: That fleas and bugs taken from plague-sick rats contain 

 B. pestis, and that some of them remain alive in, the bodies of the 

 insects from five to sixteen days;' that plague is conveyed by the bites 

 of fleas which have previously fed on the blood of animals suffering 

 with the disease ; that rat fleas bite man ; N that under experimental 

 conditions the infection is not transferred from rat to rat in the 

 absence of fleas. 



A careful study of the findings of the workers in India justifies 

 the assumption that plague is a disease of the rodent primarily and 

 accidentally, and secondarily a disease of man. An analysis of the 

 epidemiological facts collected in San Francisco leads to the same con- 

 clusion. As a result our practice with regard to suppressive meas- 

 ures and quarantine procedure has undergone a radical change in the 

 last decade. If the infection is flea-borne from rat to man in the 

 majority of cases, then the extermination of the rat should be the 

 first principle upon which to base a campaign. In the former contri- 

 bution on the subject (1907) I stated that "if we destroy the host 

 there is no longer danger of infecting the parasite." This basic 

 principle has been recognized and successfully applied in two cam- 

 paigns against plague in San Francisco. First in the outbreak in 

 Chinatown in 1903-4, and again in the larger epidemic of 1907. 



The outbreak of 1907 began May 27, a little over a year after the 

 great fire and earthquake, but no cases were discovered between that 

 time and mid-August when the disease began to appear in various 

 parts of the city. The source of infection was, in all probability, a 

 recrudescence from a focus which was not destroyed in the campaign 



