Merriam-Webster Collegiate Dictionary - vaccinia

 noun  Etymology: New Latin, from vaccinus  Date: 1803  1. a poxvirus (species Vaccinia virus of the genus Orthopoxvirus) that differs from but is closely related to the viruses causing smallpox and cowpox and that includes a strain of uncertain natural origin used in making vaccines against smallpox — called also ~ virus  2. a reaction to smallpox vaccine prepared from live ~ virus that may involve a rash, fever, headache, and body pain  • ~l adjective