This question was addressed to me in an internship interview.
Asked question is from Viral Diseases in Animals topic in portion Strategies for Enhancement in Food Production of Biology – Class 12
To explain I would say: Cowpox is a mild self-limiting eruptive skin disease of cows caused by cowpox virus. The eruption is confined to udder and teats. Inoculum from cowpox lesions was used by Jenner in 1796 as a vaccine against smallpox.