BACILLUS PESTIS. 481 



VII. PLAGUE. 



Plague was known in the second and third cen- 

 turies. In the sixth century it ravaged the Eoman 

 empire and destroyed half the population in the 

 eastern provinces. Under the name of the "black 

 death" it swept over Europe in 1347-50 with a 

 sacrifice of one-fourth of the inhabitants about 

 25,000,000. During the fifteenth and sixteenth 

 centuries many epidemics prevailed in various 

 parts of Europe, and the disease seemed to have 

 fastened itself on that part of the world. However, 

 the pneumonic form of the disease, the most con- 

 tagious, gradually became less common, or the vir- 

 ulence of the infection diminished, and this, with 

 the institution of quarantine regulations, decreased 

 the prevalence of the plague during and following 

 the seventeenth century. Nevertheless, there have 

 been occasional outbreaks in Eastern Europe since 

 that time. Following the recrudescence of plague 

 in Hongkong in 1893 and in other places later, 

 the disease has been subjected to scientific study, 

 its cause has been discovered, and the importance 

 of rigid quarantine measures at seaports in pre- 

 venting its universal extension has been proved. 



In the Hongkong epidemic of 1893-4 Kitasato 



-, , r . 1> -1 n ,1 T -, ,! 



and Yersm, working independently, discovered the organism. 

 bacillus of plague, Bacillus pestis. The organism is 

 minute (1.5 to 1.75 by 0.5 to 0.7 microns), and 

 typically is of long oval shape. The frequent oc- 

 currence of short oval cells (coccus form), longer 

 rods and distorted, swollen, vacuole-like cells (in- 

 volution or degeneration forms) signifies a high 

 degree of pleomorphism which is characteristic. 

 The longer the disease has lasted, or, on the other 

 hand, the older the culture, the more numerous are 



