Cowpox is a viral disease that normally affects the udders and teats of cows. On rare occasions, it may be transmitted to humans and produce a characteristic red skin rash and abnormally enlarged lymph nodes (lymphadenopathy). Cowpox is caused by the vaccinia virus and has been known to cause systemic reactions (generalized vaccinia) in some people who have been recently vaccinated against cowpox.
Cowpox produces immunity to smallpox and, beginning in the 19th century, the virus for cowpox was used to develop the vaccines used against smallpox. Because of the widespread vaccination, smallpox was wiped out worldwide but has now come under study again because of concern that it might be used as an agent of biological terrorism.