Monkeypox is a viral infection that resembles a very mild form of smallpox. The virus is found naturally in rodents in central and west Africa. Prior to May 2003, human monkeypox had never been identified within the Western Hemisphere. At the end of May 2003, human cases of monkeypox were identified in the United States associated with direct or close contact with prairie dogs, a Gambian giant rat, and a rabbit. Investigations identified a common distributor where prairie dogs and Gambian giant rats were housed together in Illinois. The Gambian giant rats had been imported from Ghana in a shipment containing approximately 800 small mammals, and several of these mammals tested positive for the monkeypox virus. This indicated that this shipment was the likely source of the 2003 United States monkeypox outbreak. As a result of this outbreak, the Food and Drug Administration (FDA) banned the importation of African rodents.