Fleas can be carriers of disease or may transmit parasitic worms. The most serious infection which they can spread isbubonic plague, transmitted to man by rodent fleas (Xenopsylla cheopis) which carry the causative bacillus from infected rats. In the past rodent fleas have been responsible for serious epidemics of the disease, notably the Black Death in Europe and Asia in the 14th to 17th centuries. Rodent fleas may also carry murine typhus and, because of their readiness to attack humans as well as rats, are probably the major flea carrier of disease. The dog flea is an intermediate host of the dog tapeworm , whose vertebrate host is usually the dog (occasionally the cat) but which can sometimes be transmitted to man.
Flea bites are identified as a tiny dark red spot surrounded by a reddened area. The bite persists for one or two days and is intensely irritating. First bites are not generally liable to cause serious reactions, but they may lead to hypersensitivity. Reactions are usually delayed following regular biting over a long period; there will then follow a period when reactions are immediate.