Monkeypox is a viral disease that affects animals and, in rare cases, human beings. Scientists first isolated and identified the monkeypox virus from laboratory monkeys in 1958. They obtained it from monkeys that had an unusual rash of skin pustules (small bumps filled with fluid). Historically, such pustules were called pox, leading to the name monkeypox. Other animals, including rats, squirrels, and mice, can also carry monkeypox.