No one knows where or when smallpox viruses first attacked humans. Many scientists think that Variola started out as an animal virus—perhaps of rodents or cattle—that slowly adapted to people. This is happening today with monkeypox, a disease similar to smallpox, which is spreading among people in central Africa.

Variola probably transferred to humans thousands of years ago.

Archaeologists speculate that smallpox first became widespread among humans in ancient Egypt at least thirty-five hundred years ago, striking commoners and royalty alike. The mummy of the Egyptian pharaoh Ramses V, who died in 1157 B . C ., has smallpox scars on the well-preserved skin of the face, neck, and arms.

After smallpox became endemic (firmly established) in ancient Egypt, it slowly spread around the world. Early epidemics thought to be caused by smallpox struck people in Turkey in 1346 B...