Rodents are important vectors and reservoirs of many serious human diseases. Rodents can be so numerous that, once transmission of a disease has been established in a particular area (especially in the tropics and where sanitation is poor), many or most of the human population in that area will be affected.
Rodents cause much greater harm as vectors of disease than as annoying pests; the cost to man is impossible to estimate. The list of human diseases for which rodents are either vectors or carriers of insect vectors is long and as yet incomplete. The best known include plague, salmonellosis, tularaemia and brucellosis, all caused by bacteria; the relapsing fevers, leptospirosis and rat-bite fever, caused by spirochaetes; scrub typhus, Q fever and rickettsial pox, due to rickettsia; and diseases such as certain haemorrhagic fevers, caused by arenaviruses and hantaviruses. Most of the diseases are widespread; for example leptosirosis and rat bite fevers have a worldwide distribution. Plague and murine typhus are endemic in many cosmopolitan areas.
Such rodent-borne diseases as leptospirosis and rat-bite fever are usually transmitted directly to man. In the case of many others, the disease organism reaches man via an intermediate host, usually an arthropod. Thus the intermediate host of plague is a flea; of scrub typhus, a trombiculid mite; and of the spotted fevers, a tick.
Some diseases, such as plague, are spread from one territory to another by commensal rodents and may then be passed on to field rodents of another species and lie undetected for years. Later they may find their way back to man directly or, more often, via a commensal rodent. Two factors that often make it easier for such diseases to survive for long periods are the very favourable temperature and humidity conditions of deep burrows of rodents and the very considerable anthropod fauna that these support.