Monkeypox is caused by Orthopoxvirus simiae, a virus of the genus Orthopoxvirus in the family Poxviridae. It causes disease in monkeys, but its reservoir hosts probably are squirrels and other rodents. Monkeypox is a zoonosis, it can be transmitted to humans. Disease is humans is similar to smallpox, but usually milder.
Monkeypox infection predominantly occurs in Western and Central Africa. The illegal import of sick animals poses a risk for other countries.