608 BACTERIOLOGY. 



facilitated. During epidemics, rats, mice, and flies, in 

 large numbers, become infected and die, and the disease 

 is apparently transmitted through them to man. The 

 organism is found in the feces of sick animals, in the 

 dust of infected houses, and in the soil. 



The virulence of the bacilli in cultures and in nature 

 seems to vary considerably, and is rather rapidly lost 

 when grown on artificial media. The growth in cul- 

 tures becomes more abundant after frequent transplan- 

 tation. The virulence of the organism is increased by 

 successive inoculation in certain animal species, and 

 then its pathogenic properties for other species are less 

 marked. 



Yersin, Calmette, and Borrel have succeeded in im- 

 munizing animals against the bacillus of bubonic plague 

 by inoculation, by the intravenous or intraperitoneal 

 injection of dead cultures, or by repeated subcutaneous 

 inoculation. They also succeeded in immunizing rab- 

 bits and horses, so that the serum afforded protection 

 to small animals, after subcutaneous injection of viru- 

 lent cultures, and even cured those which had been 

 inoculated, if administered within twelve hours after 

 injection. The serum has considerable antitoxic as 

 well as bactericidal properties. More recently this 

 serum has been applied by Yersin to the treatment of 

 bubonic plague in man, with very promising results. 

 Experience has shown that the treatment is more 

 efficacious the earlier the stage of the disease. When 

 treatment is begun in the first day of the attack, fever 

 and all alarming symptoms usually disappear with 

 astouishing rapidity. In cases treated at a later stage 

 larger doses of the serum are required, and even in the 

 favorable cases suppuration of the buboes is not always 



