I hope that some day the practice of producing cowpox in human beings will spread over the world - when that day comes, there will be no more smallpox.

This quote is about the practice of using a weaker form of a virus, called cowpox, to protect humans against a more serious virus called smallpox. If this practice, known as vaccination, were to become widely used, then smallpox would no longer occur because people would be protected against it.