Smallpox is an acute infectious disease caused by a virus.
Smallpox infects only humans, and the last naturally acquired case of smallpox in the world occurred in 1977. No one has naturally contracted smallpox since 1977. When smallpox naturally occurred, the disease was highly contagious. People who developed a cough when they had smallpox could spread the disease by coughing and were considered at very high risk of transmitting their disease to others. The initial symptoms of smallpox included the acute onset of fever, chills, headache, nausea, vomiting and severe muscle aches.