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'Natural reservoir' or 'nidus', refers to the long-term host of the pathogen of an infectious disease. It is often the case that hosts do not get the disease carried by the pathogen or it is asymptomatic and non-lethal. Once discovered, natural reservoirs elucidate the complete life cycle of infectious diseases, providing effective prevention and control. Examples of natural reservoirs are:

★ Field mice, for hantaviruses and Lassa fever

★ Marmots, black rats, prairie dogs, chipmunks and squirrels for bubonic plague

★ Armadillos and opossums for Chagas disease

★ Ticks for babesiosis and Rocky Mountain spotted fever

★ Ground squirrels, porcupines, and chipmunks for Colorado tick fever

★ Mosquitoes for filariasis and malaria

★ Snails for schistosomiasis and swimmer's itch

★ Pigs for cestode worm infections

★ Raccoons, skunks, foxes and bats for rabies

★ Shellfish for cholera

★ Fowl (ducks and geese) for avian flu
Some diseases have no non-human reservoir: poliomyelitis and smallpox are prominent examples.
The concept of natural reservoir could also be extended to asymptomatic human bearers of the infecting agent, exemplified by the famous case of Typhoid Mary.
The natural reservoir of some diseases remain unknown. This is the case of the Ebola disease, which is caused by a virus.