80 BACTERIOLOGY. 



stances, and may be used to develop a passive im- 

 munity in other animals. 



THE BACILLUS OF PLAGUE (BACILLUS PESTIS). 



The bacillus of bubonic plague was discovered by 

 Kitasato and Yersin during the epidemic in China in 



1893. It is a short, thick bacillus with rounded ends. 

 In old cultures atypical forms are found, some like 

 cocci, others club-shaped like the diphtheria bacillus. 

 It stains more deeply at the ends than in the center. 

 It is not motile and does not form spores. It will 

 grow only .in the presence of oxygen. In dark, moist 

 places the organism will live for months or years. In 

 the sputum and pus from patients it lives for one or 

 two weeks. In cadavers they may live for several 

 weeks. Dry heat destroys the bacillus in one hour, 

 boiling in a few minutes. Direct sunlight requires 

 four or five hours. Carbolic acid (5 per cent.) and 

 bichloride of mercury ( I : 1000) destroy them, in ten 

 minutes. 



The plague raged from the sixth to the seven- 

 teenth century, and in the fourteenth century the 

 black death, as it was called, destroyed one-quarter of 

 the population of Europe. The great plague in Lon- 

 don in 1665 destroyed 70,000 people. The disease 

 subsided then and remained practically dormant until 



1894, when it broke out in Hong Kong. It spread 

 thence to other countries, and a small epidemic oc- 

 curred in San Francisco in 1907. 



The infection may enter through the skin or by 



