Yersinia pestis, the organism which causes plague in man and animals, circulates from host to host in nature by means of one of many flea vectors. There are about 340 mammals known to be susceptible to plague and over 30 fleas which are known to carry the organism. The habits of flea and host, usually a rodent, must to some extent coincide before infection can be established, and these habits are affected by the temperature and humidity of the environment, by vegetation cover, by season, indeed by everything that is included in the term 'landscape epidemiology'. The ease with which fleas either block or not after feeding is important, but sheer numbers in a focus may sometimes be more important. Some rodents are so highly susceptible that whole colonies may be wiped out by plague, yet alongside these susceptible animals there are usually other resistant hosts which maintain the infection in an area between epizootics.
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