BACILLUS PESTIS 



237 



relationship between the members of the group. So far as is known 

 none of the organisms affecting the lower animals are pathogenic 

 for man except a very similar organism, B. Pestis, which is respon- 

 sible for the much-dreaded bubonic plague or " black death." 



B. Pestis. Records of the ravages of the bubonic plague have 

 been handed down through the centuries. At intervals it has 

 appeared in vast epidemics and so great has been the infectiousness 

 and mortality of the disease that in congested districts whole 

 populations have succumbed. The " Great Plague " of the 

 fourteenth century spread over all Europe, and so frightful was 

 its severity that one quarter of the population or about 25,000,000 

 persons perished. Commerce 

 was suspended and people fled 

 panic stricken from the towns 

 to the open fields for safety. 

 During the last two centuries 

 Western Europe has been prac- 

 tically free from the disease. 

 It still occurs, however, in all 

 its horror in India, the annual 

 mortality averaging about 

 500,000. It is thought that 

 the disease made its first ap- 

 pearance in America in 1899 at 

 Santos, Brazil ; since then other cases have been reported in San 

 Francisco, Mexico, and Central America. 



The causal agent of the disease, B. pestis, was discovered simul- 

 taneously by Kitasato and Yersin in 1894 during an epidemic in 

 China. A number of accidental infections with pure cultures 

 have established its specificity. 



Morphology and Staining. In film preparations from infected 

 tissues the bacilli appear as short, thick rods with rounded ends 

 about 1.6 /* in length and 0.6 p in width. In body fluids they 

 may be seen singly or in pairs and rarely in chains ; in broth cul- 

 tures, on the contrary, they remain attached, and the individual 

 organisms are so short and thick as to give almost the appearance 



FIG. 32. Bacillus Pestis. 



