THE BACILLUS OF BUBONIC PLAGUE. 315 



in fatal infection in gray and white rats, house- and 

 field-mice, guinea-pigs, rabbits, hogs, apes, cats, chick- 

 ens, sparrows, and flies. He also calls attention to the 

 fact that flies may live for several days after being in- 

 fected with this organism, and if at liberty to fly about 

 may infect persons or food-stuffs on which they alight 

 or fall. 



Recent investigations in India lead to the belief that 

 the flea is one of the most common and important of the 

 agents of transmission, carrying the disease from man to 

 animals (rodents, rats in particular) and from animals 

 to man. 



The bacilli apparently lose their virulence after 

 long-continued cultivation under artificial conditions, 

 and it is said that from slowly developing, chronic 

 buboes non-virulent or feebly virulent cultures are 

 often obtained. Variations in the degree of virulence 

 have been observed in different colonies from the same 

 source. Virulence is said by Yersin, Calmette, and 

 Borrel 1 to be accentuated by passing the organism 

 through a series of susceptible animals. 



In man the bacilli are most numerous in the en- 

 larged, suppurating lymphatics. They are present, but 

 in smaller numbers, in the blood and the internal 

 organs. 



It has been observed that in the suppurating lym- 

 phatic glands of man a variety of organisms may be 

 present, but among them are always the plague bacilli. 

 Occasionally, micrococci predominate. In these cases 

 of mixed infection the pest bacilli are said to stain less 

 intensely with alkaline methylene-blue than do the 

 streptococci, and more intensely than do the micrococci 



i Anuales de 1' institut Pasteur, 1895, p. 588. 



