How can we be sure?
Abstractlt is difficult to prove that a disease no longer exists on earth. But a mass of carefully compiled evidence has convinced the Global Commission that Smallpox Target Zero has in truth been attained
Fenner, Frank. (1980). How can we be sure?. World Health;1980 May p.35-40; https://apps.who.int/iris/handle/10665/202498
JournalWorld Health;1980 May p.35-40;