cowpox, infectious disease of cows caused by a virus related to the virus of smallpox. Also called variola, it is characterized by pustular lesions on the teats and udder. Cowpox is transmitted by contact, inducing a mild infection of the hands in persons who milk infected cows. The fact that such persons had immunity to smallpox led Edward Jenner to attempt vaccination with this virus, instead of using the dangerous method of vaccinating with material from the sores of smallpox. Jenner's method was successful and is the basis of the modern vaccination against smallpox. Horses and sheep may contract a similar disease.