Infection is enzootic among squirrels and monkeys in the rainforests of western and central Africa .
In 1996, cases of monkeypox were reported from villages in Zaire. The WHO and CDC investigated and found 511 monkeypox cases since February of that year. This monkeypox outbreak was the biggest recorded to date. Vaccinia virus, that which proved so effective in the eradication of smallpox, allowed for a new approach to the development of vaccines. Now, widespread research is being conducted in which genes coding for the surface proteins of a given virus are inserted into the genome of an avirulent one.