Infection with the monkeypox virus is responsible for the extremely rare disease known as monkeypox. Smallpox and monkeypox viruses are both members of the variola virus family. The symptoms of monkeypox are similar to those of smallpox, but they are less severe, and the disease itself is rarely fatal. The monkeypox virus is not linked to the chickenpox virus. In 1958, researchers uncovered monkeypox after two colonies of captive monkeys experienced epidemics of a pox-like disease.