Not so with smallpox. To create this vaccine, you begin with another virus that is similar to the smallpox virus, yet different enough not to bring on the smallpox disease once it enters your body.

Smallpox, or variola, virus has killed more human beings than any other infectious agent. Fortunately, smallpox virus has only one host-humans -- which made it possible to eradicate it. In 1967, the ...