Smallpox is an acute contagious disease and was one of the world's most feared sicknesses until it was officially declared eradicated worldwide in 1979.

"WHO takes any report of smallpox seriously," Gregory Hartl, a spokesman for the Geneva-based United Nations health agency, told Reuters via email.

"WHO is aware of the reports coming out of Uganda and is taking all the necessary measures to investigate and verify."   He added that the WHO had received reports before of smallpox cases, but they had turned out to be false alarms.

"In the past, these cases have always turned out not to be smallpox and were, usually, either chickenpox or monkeypox cases," he said, stressing that these reports were of suspected cases of the disease.

Smallpox was eradicated by a collaborative global vaccination program led by the World Health Organization. The last known natural case was in Somalia in 1977.

Since then, the only known cases were caused by a laboratory accident in 1978 in Birmingham, England, which killed one person and caused a limited outbreak.