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 
astovishing 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 
