How did the world wipe out smallpox, and why only smallpox?

With a vaccine, a global campaign that tracked down each outbreak, and a disease that had few places to hide. Smallpox once killed about 30% of the people it infected. During the 20th century it is estimated to have caused 250-500 million deaths. In 1967 the World Health Organization estimated 15 million cases and two million deaths in a single year, and that same year it intensified its efforts. In May 1980 it certified the global eradication of smallpox.

The weak points are biological. Smallpox was not known to be transmitted by insects or animals, and there was no asymptomatic carrier state: nobody could carry it without showing it. It also needed close contact, and transmission came after the rash appeared, a visible sign.

The method was ring vaccination: when someone falls ill, vaccinate their contacts and the contacts of those contacts, creating a ring of protection that can limit the spread. It requires thorough and rapid surveillance and case investigation, and the Intensified Smallpox Eradication Program used it with great success.

The last naturally occurring case was diagnosed in October 1977. Eradication was certified in 1980, making smallpox the only human disease to have been eradicated. It is one of two infectious diseases eradicated; the other is rinderpest, declared eradicated in 2011.