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.