PATHS AND MODES OF INFECTION. 443 



sage of the bacilli into the respiratory passages. Nevertheless 

 there must be certain factors, still imperfectly understood, which 

 determine the incidence of this form; as in some epidemics of 

 the highest virulence plague pneumonia has been practically 

 absent, though opportunities for infection by inhalation must 

 have been present. On the other hand, a case of plague pneu- 

 monia is of great infectivity in producing other cases of plague 

 pneumonia. 



With regard to the mode of infection it may be stated, in the 

 first place, that if we except plague pneumonia and those rare 

 conditions in which true plague eruptions are present, direct 

 infection from patient to patient is relatively uncommon. This 

 is in accordance with the fact that in bubonic plague the bacilli 

 are not discharged from the unbroken surface of the body, and 

 are only present in the secretions in severe cases. In the great 

 majority contagion is of indirect character, through the medium 

 of soiled clothes, f omites — in fact, contaminated articles gener- 

 ally ; thus rooms and houses come to be " sources of infection." 

 In addition to cases of the disease in the human subject, the 

 affection in animals, especially in rats, plays an important part 

 in the spread of epidemics. The disease in these animals has, 

 in fact, been the means of rapidly distributing infection over 

 wide areas of a town or district. This has been abundantly 

 proved in the case of Bombay, where observations have shown 

 that the migration of plague-infected rats to quarters compara- 

 tively free from the disease has been followed by extensive out- 

 breaks in these places. On the other hand, there is no doubt 

 that plague in a very widespread and virulent form may occur 

 without the agency of these animals. 



The view that fleas and other insects play a part in the spread 

 of plague has obtained pretty wide acceptance, but recent exami- 

 nation has proved that most of the data on which this is based 

 are of unsatisfactory nature. Yersin found the plague bacillus 

 in the dead bodies of these insects, and Simond brought forward 

 the results of experiments which appeared to show that the fleas 

 infecting a plague-stricken rat left the animal when it died, and 

 might produce plague in a healthy animal. Experiments per- 

 formed by others have, however, given negative results, and the 

 finding of the Indian Commission is that suctorial insects are 

 practically of no importance as transmitters of infection. 



