Smallpox was once one of the most feared diseases in the world - disfiguring and often deadly. Donald Henderson is the American doctor who led the fight to rid the world of this terrible disease. His campaign started in the 1960s, and international scientists only agreed that he and his team had succeeded on 9 December, 1979. He tells Witness how they tackled virus - in the laboratory, and on the ground. Image: Smallpox cell, Credit: Getty Images