FLEAS AND PLAGUE 411 



14th century in Europe one-fourth of the population of that 

 continent, or 25 million people, died of the disease. Superstition 

 and unreasoning terror led to horrible persecution and torture of 

 innocent people who were supposed ?o cause the plague. At 

 present the disease is practically confined to tropical countries, 

 and is especially prevalent in India, where an average of about one 

 million deaths a year are caused by it. The practical disappear- 

 ance of plague from Europe is thought by some authors to be 

 associated with a change in the rat fauna of Europe, the do- 

 mestic and gregarious black rat, Epimys rattus, being replaced 

 by the wilder and more scattered brown rat, Epimys norvegicus. 

 The disease, however, has often been introduced from the tropics 

 into other countries, and there is constant danger of this wherever 

 the strictest preventive regulations are not enforced. In 1900 

 the disease was introduced into San Francisco, and there is every 

 reason to believe that had knowledge of preventive medicine 

 been at the point where it was 300 years ago, the United States 

 would have been swept as was Europe in the 14th century. In- 

 stead, knowing that rats were the chief reservoir of the disease, 

 and that rat fleas were instrumental in transmitting the disease 

 from rat to man, the U. S. Public Health Service took hold of the 

 situation and instituted an anti-rat campaign such as had never 

 been thought of before. Over a million rats were caught, ex- 

 amined and destroyed in the city of San Francisco. The infec- 

 tion spread, however, and became established in the ground 

 squirrels of several counties in California. From July 1, 1913, to 

 November, 1914, over 20,000,000 ground squirrels were destroyed 

 in infected districts in California. So strenuous were the efforts 

 to stamp out the disease before it could get beyond control that 

 only 187 cases of the disease in man occurred in California, 

 with none since 1914. New Orleans has also had a taste of plague, 

 and infected rats have been taken in the vicinity of the water 

 front in Seattle. 



The steps which have made possible an intelligent fight against' 

 plague were the discovery of the plague germ, Bacillus pestis, 

 by Yerson in 1894, the establishment of the identity of the disease 

 with that of rats by the same worker, the discovery of the mul- 

 tiplication of the plague germs in the gut of rat fleas, Xeno- 

 psylla cheopis, by Listen in 1905, and finally conclusive experi- 

 mental proof by the British Plague Commission in India in 



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