Monkeypox is an infectious viral disease that can occur in both humans and some other animals. Symptoms include fever, swollen lymph nodes, and a rash that forms blisters and then crusts over. The time from exposure to onset of symptoms ranges from 5 to 21 days. The duration of symptoms is typically 2 to 4 weeks. There may be mild symptoms, but to what extent it may occur without any symptoms is not known. The classic presentation of fever and muscle pains, followed by swollen glands, with lesions all at the same stage, has not been found to be common to all outbreaks. Cases may be severe, especially in children, pregnant women or people with suppressed immune systems. The disease is caused by monkeypox virus, a zoonotic virus in the genus Orthopoxvirus. The variola virus, the causative agent of smallpox, was also in this genus. Of the two types in humans, the West African type causes a less severe disease than the Central African type.