Cowpox is a disease. This disease affects the skin. It is caused by a virus (cowpox virus) that is related to the Vaccinia virus. People (or animals) who have the disease have red blisters. The disease can be spread by touch from cows to humans. The virus that causes cowpox was used to perform the first successful vaccination against another disease. The disease vaccinated against was the deadly smallpox. Smallpox is caused by the related variola virus. Therefore, the word "vaccination" has the Latin root vaca meaning cow.