The Threat   Throughout the last three thousand years, smallpox has shadowed civilization. A viral infection, the disease spread along trade routes, emerging first in Africa, Asia and Europe and reaching the Americas in the sixteenth century. Because smallpox requires a human host to survive it tended to smolder in densely populated areas, erupting in a full-blown epidemic every ten years or so. Wherever it appeared, the legacy of smallpox was death, blindness, sterility and scarring.     While some medical practitioners claimed to cure smallpox, most medical traditions focused on prevention. Quarantining smallpox patients often limited the spread of the disease and was commonly used even into the twentieth century as there is no cure for smallpox.