Rodents are of public health importance as reservoirs of numerous diseases of man, and of economic importance because of their destruction of growing crops and stored food. Rats are difficult to catch or poison. Although they are not particularly physiologically resistant to poisons, they cannily avoid eating unfamiliar food, or anything which they suspect as being harmful. In addition, single dose rodenticides have tended to be too persistent for widespread use; the body of the dead rat is itself toxic for other animals. The alternative cumulative poisons are anticoagulants, but anticoagulant resistances have been increasing.
Almost all countries within the temperate and tropical zones are engaged in active rodent control. The damage done by rodents to man's food, his animals and other property (including structural damage by gnawing through wood pipes and cabling) is extensive, and the list of human diseases transmittable by these animals or their parasites is not yet complete. The best known, but not necessarily the most important, include plague, salmonellosis, tularaemia and brucellosis, the relapsing fevers, leptospirosis and rat-bite fever, all caused by bacteria; scrub typhus, Q fever and rickettsial pox, due to rickettsiae; and diseases such as certain haemorrhagic fevers, caused by viruses about which little is known at present. In the USA it has been estimated that up to 20% of unexplained fires may be started by rats or mice biting through electricity cables.