240 BACTERIOLOGY FOR NURSES 



mainly through the agency of the flea. On the other hand, the 

 pneumonic form is frequently transmitted from one person to 

 another by careless disposal of the sputum or droplet infection. 

 The mode by which the bacilli enter the body does not necessarily 

 determine the type of infection which will ensue. 



The above three types of the disease are usually classified as 

 pestis major; milder forms occur known as pestis minor, which bear 

 somewhat the same relationship to the severer form that " walk- 

 ing typhoid " does to typhoid fever. 



Modes of Infection. When first it was noticed that an epi- 

 demic of plague was accompanied by an increased mortality 

 amongst rats a supposition at once arose that there must be some 

 relationship between rat plague and human plague, and the possible 

 mode of the conveyance of the disease from rat to man or from rat 

 to other rodent became a question of prime interest. As a result of 

 experiments it soon became evident that rat fleas were responsible 

 for the transfer of the infection. It was noticed that infected 

 animals might be placed in a cage with healthy animals without 

 the latter contracting the disease if fleas were excluded; on the 

 other hand, healthy animals placed near enough to flea-infested 

 plague rats invariably contracted the disease. It was also found 

 that the common rat flea infests and bites human beings; large 

 numbers were found on the legs of men who entered for a short 

 time plague-infected houses. The ease with which bacilli may 

 enter the tissues was also demonstrated by allowing a non-infected 

 flea to bite a rat and then placing a drop of culture over the bite 

 after which infection resulted. Since the blood of an infected 

 rat may contain as many as 100,000,000 bacilli per c.c. an enormous 

 number may be present even in the amount of blood sucked by a 

 flea. The organisms multiply further in the stomach of the in- 

 sect, which may remain infected for one or two months. Trans- 

 ference of the organism to man probably takes place by means of 

 the feces expelled by the insect while feeding or the regurgitation 

 of infected blood previously taken into the stomach. Once de- 

 posited on the skin it is an apparently simple matter for the bacilli 

 to enter the skin through the minute opening made by the flea 



