Why did the smallpox vaccine end a disease that had killed millions?

Smallpox was one of history’s deadliest diseases, but it had a crucial weakness: it spread only between humans. That made it possible to break transmission with ring vaccination, surveillance, and targeted containment. The World Health Organization’s intensified eradication campaign helped drive the last naturally occurring case in 1977, and smallpox was officially declared eradicated in 1980. The success depended on an effective vaccine, strong public-health coordination, and the lack of any nonhuman reservoir. It remains the only human disease eradicated by vaccination.