Vaccinia

Vaccinia is the virus that causes cowpox. It is in the pox family of viruses, which also includes smallpox.

Vaccinia is so mild that it is typically asymptomatic in healthy individuals, but may causes a mild rash and fever, with an extremely low rate of fatality.

The body's immune response to vaccinia also immunizes against smallpox, and for this reason, vaccinia is used as a live-virus innoculator against smallpox. Vaccinia smallpox vaccine does not contain and cannot cause smallpox, but approximately one in one million individials will develop a fatal response to the vaccination.