NON-SPORING BACILLI 259 



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avoids man as far as possible, and finds its food in sewers, 

 ditches, fields, etc., rather than in inhabited houses. 

 Mus rattus, on the other hand, is a domestic animal in 

 India, and lives in close and intimate association with the 

 home-life of the people. The Mus rattus, therefore, is 

 chiefly responsible for the transmission of the disease to 

 man ; while the Mus decumanus is of special importance 

 in maintaining the disease from season to season. The 

 Indian Plague Commission conclude that plague is a rat 

 disease, having a regular periodicity, namely (a) an epizootic 

 season, from December to May inclusive, and (b) a non- 

 epizootic season, from June to November inclusive. 

 During the latter period there are few cases of plague in 

 rats, fleas are scanty (this is given as a cause of the decrease 

 in cases of plague in rats), and in some villages where the 

 Mus rattus alone prevails, plague may actually die out 

 each season. The Mus decumanus is more infested with 

 fleas, and is thought to keep the infection going from 

 season to season. A fresh epizootic first chiefly affects the 

 Mus decumanus, then spreads to Mus rattus, and then to 

 human beings. In this way are explained the outbreaks 

 of plague in India, year after year since 1896, causing 

 nearly a million deaths in 1904 among the natives, while 

 the attendants and Europeans enjoy almost complete 

 immunity, although both hospitals and camps abound 

 with Pulex irritans. This is ascribed to the habits of this 

 common flea of man, in that it rarely bites other creatures 

 than man, and that in man with plague the blood is not 

 so alive with bacilli as in the rat. The chances of infec- 

 tion by Pulex irritans are from these causes enormously 

 reduced. 



2. By inhalation of the virus, causing the pneumonic 

 form of the plague, which is very rapid and fatal. It is 

 not understood what factors determine the appearance of 

 the disease in this form, but once started it is extremely 

 infectious. The symptoms are very similar to those of 

 acute lobar pneumonia (although the pneumonia is mainly 

 lobular in its distribution), with high fever, rapid respiration, 

 and haemorrhagic sputa. The sputum contains the bacilli 

 in enormous numbers, and almost in pure culture. In 

 Egypt, the summer type is bubonic, and the winter type 

 pneumonic. 



