364 THE WONDER OF LIFE 



squirrels. It causes an acute fever associated with swellings 

 of the lymphatic glands (bubonic type), or it may primarily 

 attack the lungs (pneumonic type), or it may primarily 

 poison the blood (septicsemic type). The ' black death ' 

 which destroyed about a fourth of the population of 

 Europe in the fourteenth century was apparently of the 

 pneumonic type and highly infectious. 



The microbe of plague (in its ordinary modem form) 

 is not effectively transported by wind or in water or in 

 food. In rare cases it might be swallowed by man, but it 

 cannot make an effective entrance through the food canal. 

 It enters man through the bite of one of the Indian 

 rat fleas (Pulex cJieopis). An outbreak of plague among 

 human beings in India is preceded by an outbreak of 

 plague among the black rats {Mus rattus) which frequent the 

 houses in great numbers. A flea bearing the plague baciUi 

 from the rat's blood bites man and thus infects him. There 

 are other kinds of fleas on rats, but Pulex cheopis is the only 

 one which will readily bite man. 



A plague is known to occur in the marmots or ' tarba- 

 gans ' of Manchuria and an analogous disease in those who 

 hunt the animal for the sake of its skin. There are very 

 large fleas on the marmot, and it is possible that in the 

 epidemics of plague in Manchuria the marmot-flea may 

 play the same part as the rat- flea in India. 



There have been many hints lately that mites have a 

 more comphcated inter-relation with man and his domestic 

 animals than that which is implied in their being a 

 punishment for lack of cleanhness. (For mites are always 

 trying to clean things up.) It is probable that they have, 

 hke ticks, a role in the spreading of disease. It has been 

 suggested that the very common follicle-mite {Demodex 



