www.bbc.co.uk/science/hottopics/smallpox/vaccine.shtml
"In 1939, British scientist Allan Downie discovered that the smallpox vaccine widely in use was not cowpox as everyone had thought, but another type of 'orthopoxvirus'. It came to be called vaccinia virus. The origin of this virus, and whether it had been accidentally created in the laboratory or had existed in nature, is unknown."