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XX] dry. Pneumonic plague, not being spread by the flea, is not influenced by temperature in this way (see p. 344).

Bionomics of the rat-flea.—In ordinary circumstances the rat-flea completes its developmental cycle in fourteen days to three weeks, but in warm, damp weather this may be shortened to ten days. The average life of a flea, apart from its host, is about ten days, but it is capable of remaining alive without food for two months, should the temperature of the

Tropical Diseases - Fig 69.png

air be low at the time. In tropical temperatures the insect can harbour the plague bacillus without feeding on blood for forty-five days.

Apart from the very serious danger arising from vermin affected with chronic plague, which may hang about a house for a long time, it would appear that the house itself does not retain the infection for any length of time. The Plague Commission has shown that floors of cow-dung if contaminated with Bacillus pestis do not remain very infective for more