THE PLAGUE 119 



sputum deposited on clothing may retain its vitality for months. The 

 second is that air-borne infection does not play an important role in 

 the distribution of this disease. The last statement must not be taken 

 as denying the possibility or even the probability of droplet infection, 

 especially in pneumonic cases. Direct sunlight kills speedily, the 

 time varying with the thickness of the layer. Speaking generally, 

 hours are necessary. An agar culture exposed to direct sunlight con- 

 tains virulent bacilli after two hours and bubonic pus placed on glass 

 and exposed directly to sunlight is non-virulent after about six hours. 

 Bouillon cultures used for immunizing purposes are sterilized by heat- 

 ing for one hour at 58 C. (136.4 F.). Boiling kills in one minute. 

 Dry heat at 100 C. continued for one hour kills the bacillus. Ordinary 

 methods of disinfection suffice to destroy this organism. 



The virulence of this bacillus is quite variable. It is probably true 

 that the bacterium from any animal dead of this disease is pathogenic 

 to the rat and guinea-pig, but some strains retain their virulence 

 through many generations when grown artificially, while others soon 

 lose it. The virulence is more quickly lost at incubator than at lower 

 temperature. In order to preserve the virulence of cultures, passage 

 through susceptible animals should be resorted to from time to time. 

 Repeated animal passage may revive a failing virulence. 



All rodents are susceptible to the plague bacillus, but between the 

 different species of these there are wide variations. The most sus- 

 ceptible is the rat, which suffers extensively from epidemics of this 

 disease and responds to every form of artificial inoculation. When the 

 plague virus is rubbed into the shaved or clipped abdomen of this 

 animal the bacillus finds its way through the skin, apparently when 

 there is no injury, and causes a general infection. When the skin is 

 pricked with the finest infected needle, the nearest glands become 

 swollen and the neighboring tissue edematous, while the infection 

 extends to other glands and soon a general infection is established. 

 The spleen is enlarged, the lungs and liver become hyperemic and 

 after death the bacillus is found in every part of the body. When the 

 injection is made into the peritoneal cavity, the bacillus passes through 

 the peritoneal walls, which seem only somewhat more moist than nor- 

 mal, and reaches the various organs. Given by mouth, the first visible 



