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Yersinia pestis is a Gram-negative pleomorphic bacterium, today mainly limited to animals but is occasionally seen to cause disease even to the point of sporadic epidemics in humans.
The principal reservoirs for this infection are woodland rodents. These rodents can transmit the bacterium to urban rat populations. The rat flea, Xenopsylla cheopic, acts as the vector.
These rat fleas may bite humans when there is a drop in the rat population and so infect them. Also, humans may become infected via wound contact with infected faeces or via droplet inhalation.