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Cowpox is a disease of the skin caused by the Vaccinia virus. The ailment manifests itself in the form of red blisters and is transmitted 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, which is caused by the related variola virus. Therefore the word "vaccination" has the latin root vaca meaning cow.

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In 1798 the rural English physician Edward Jenner made a curious observation. His patients who had contracted and recovered from cowpox, a disease similar to but much milder than smallpox, seemed to be immune not only to further cases of cowpox, but also to smallpox. By scratching the fluid from cowpox lesions into the skin of healthy individuals, he was able to immunize those people against smallpox.