Plague seems at times to vary widely in its virulence and the extent to which it infects a host. These fluctuations in morbidity (infection rate) may be an evolutionary response to ensure the survival of the bacillus. When a rat's bloodstream is massively clogged by Yersinia pestis, the possibility increases dramatically that any uninfected flea which bites it will ingest bacteria. In the usual, slower course of the disease those chances are actually quite slim, because there is normally a low level of Yersinia pestis in the victim's bloodstream. Thus, the occasional, swift course this disease takes is most likely a measure designed to help the bacterium across a critical juncture in its life cycle, the leap from the bloodstream of its mammalian host to the digestive tract of a rat flea.

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