Smallpox is the only human disease we have wiped out

Smallpox killed hundreds of millions of people in the twentieth century alone. In 1980 the world was declared free of it. Here is how that was done.

For most of history, smallpox was one of the deadliest diseases people knew. Its most common form, variola major, killed about three in ten of those it infected.

Estimates for the twentieth century alone run to hundreds of millions of deaths. As late as 1967, the World Health Organization estimated that about 15 million people caught it in a single year, and some 2 million died.

The tool to stop it was already old. In 1796 the English doctor Edward Jenner, who had noticed that milkmaids who caught cowpox seemed protected from smallpox, showed that material from cowpox could protect people. It was the first successful vaccine.

Finding every case

In 1967 the WHO launched an intensified eradication programme, with a better freeze-dried vaccine, a simple two-pronged needle and, above all, close surveillance. Teams hunted for cases, isolated them and vaccinated the people around them, cutting each chain of infection before it could spread.

Smallpox had features that helped. It passed only between people, not through animals or insects. It caused a visible rash, and there were no hidden carriers who felt well while spreading it. Once a case was found, the outbreak had few places left to hide.

The last natural case was Ali Maow Maalin, a hospital cook in Merca, Somalia, who fell ill in October 1977 and recovered. Health workers traced his contacts, vaccinated tens of thousands of people in the area and searched for months. No further case turned up. He later worked in Somalia's campaign against polio.

One more death followed. In 1978 Janet Parker, a medical photographer at the University of Birmingham medical school in England, caught smallpox from virus grown for research in a laboratory there. She died in September that year, the last person known to have died of the disease.

A global commission certified the eradication in December 1979, and on 8 May 1980 the World Health Assembly declared the world free of smallpox. Samples of the virus are still kept in two secure laboratories, one in the United States and one in Russia. Only one other infectious disease, rinderpest, which struck cattle and related animals, has since been declared eradicated.