Smallpox, or variola (from Latin varus meaning pimple), is a highly contagious viral disease, one of the very worst to afflict humankind since time immemorial. It is believed to have originated in northeastern Africa with the first agricultural settlements--around 10,000 B.C., spreading to India via Egyptian merchants, and was known in China in ancient times. Several early historians described plagues with the characteristic symptoms of smallpox (chills, headache, backache, nausea, vomiting, fever, and red spots on the skin that develop into pus-filled blisters). Greek historian Thucidydes (died c. 326 B.C.) chronicled a disease that broke out during the Peloponnesian War in the fifth century B.C., noting that a person who contracted this disease and survived was called "fortunate and happy," since he could never get the disease again. The early Christian historian, Eusebius (died c. 379 B.C.), writing in the third century, referred to an epidemic (thought...