The cowpox virus was used to perform the first successful vaccination against another disease, smallpox, which is caused by the related Variola virus. Therefore, the word "vaccination" — first used by Edward Jenner (an English physician) in 1796 — has the Latin root vacca meaning cow, or from Latin root vaccinia meaning cowpox.
The virus is found in Europe, and mainly in the UK. Human cases today are very rare and most often contracted from domestic cats. The virus is not commonly found in cows; the reservoir hosts for the virus are woodland rodents, particularly voles. It is from these rodents that domestic cats contract the virus. Symptoms in cats include lesions on the face, neck, forelimbs, and paws, and less commonly upper respiratory tract infection. Symptoms of infection with cowpox virus in humans are localized, pustular lesions generally found on the hands and limited to the site of introduction. The incubation period is nine to ten days. The virus is prevalent in late summer and autumn.
In a famous letter to Meriwether Lewis in 1803, Thomas Jefferson instructed the Lewis and Clark expedition to "carry with you some matter of the kine-pox; inform those of them with whom you may be, of its efficacy as a preservative from the smallpox; & encourage them in the use of it... Jefferson had developed an interest in protecting Native Americans from smallpox having been aware of epidemics along the Missouri River during the previous century. One year prior to his special instructions to Lewis, Jefferson had persuaded a visiting delegation of North American Indian Chieftains to be vaccinated with kinepox during the winter of 1801-2. Unfortunately, Lewis never got the opportunity to use kinepox during the pair's expedition as it had become inadvertently inactive — a common occurrence in a time before vaccines were stabilized with preservatives like glycerol or kept at refrigeration temperatures.
Cowpox was the original vaccine of sorts for smallpox. After infection with the disease, the body (usually) gains the ability of recognizing the similar smallpox virus from its antigens and so is able to fight the smallpox disease much more efficiently. The vaccinia virus now used for smallpox vaccination is sufficiently different from the cowpox virus found in the wild as to be considered a separate virus.