Cowpox is a contagious viral disease of cows. It is characterized by small pustules (pus-filled bumps) on the skin, especially on the cows' udders and teats. Cowpox can be passed on to human beings, though for people it is a minor disease that causes a few sores on the hands. Cowpox helped Edward Jenner, a British physician, discover vaccination as a means of preventing smallpox, an extremely contagious disease among people.