They paid a thousand dollars to anyone who reported a smallpox case

Smallpox killed some three hundred million people in the twentieth century alone, more than both world wars put together, and it is the only human disease we have managed to wipe off the Earth. It was declared extinct on 8 May 1980, at the 33rd World Health Assembly, the annual meeting of the world's health ministries.

In the campaign's final years, the World Health Organization stopped spending its money on vaccinating everybody and started spending it on finding patients: it paid teachers, barbers and market traders to report cases, and came to offer a thousand dollars — around five thousand in today's money — to anyone who reported a confirmed case. What follows runs from the end to the beginning, down to the day somebody thought of it.

December 1979. Before that declaration comes a commission: independent scientists from abroad who spend two years going through the countries that had been endemic — hospitals, markets, villages, school registers — so as to be able to sign that not a single case is left. A disease is not declared extinct when it stops appearing: it is declared when outsiders have searched for two years and found nothing.

Birmingham, September 1978. Janet Parker, a medical photographer at the University of Birmingham Medical School, works in a darkroom directly above a laboratory that still holds live samples of the virus. She falls ill in the middle of August and dies on 11 September. She is the last person in the world to die of smallpox, and by then the disease was no longer circulating in any street: a laboratory accident took her. From that case came the decision to gather the remaining samples in two guarded freezers: since 1984 the one at the CDC, the American public health agency, in Atlanta, and since 1994 the one at the Vector centre in Koltsovo, Siberia. There they remain, inventoried by international agreement.

Merca, Somalia, 26 October 1977. Ali Maow Maalin, twenty-three, a cook at the port hospital, turns up with a fever and a rash. First he is diagnosed with malaria; then with chickenpox. It is a tracker from the vaccination campaign who, on seeing him, recognises what is in front of him: variola minor, the less lethal of the two forms. Maalin is the last case of smallpox contracted naturally in the history of the species.

And he had worked as an occasional vaccinator in that very campaign: he had given the jab to his own neighbours. Nobody gave it to him. He said afterwards that he had dodged his own vaccination because he was afraid of needles, and that he had assumed that, working where he worked, nothing would happen to him. He survived. In the fortnight after the diagnosis, the teams tracked down his contacts one by one, vaccinated everyone living around the hospital and posted watchers door to door across the town. Not one further case appeared.

Maalin later became local coordinator of the polio campaigns in his region and spent years persuading, house by house, families who did not want their children vaccinated; he used his own story as the argument. He died of malaria in July 2013, in the middle of a campaign.

Bhola Island, Bangladesh, October 1975. Rahima Banu, three years old, is the last natural case of variola major, the variant that killed one patient in three. It is not a doctor who finds her: she is reported by an eight-year-old girl from the village who had heard that there was money for reporting a case, and who was paid two hundred and fifty taka for it, several weeks' wages for a day labourer in the delta. The campaign had learned that buying the information came cheaper than looking for it, and three years later, to hunt down the last cases on the planet, the reward was a thousand dollars. Rahima Banu survived and is still alive.

India, 1975. The country's last case is recorded in mid-May, and getting there took the largest operation of the whole campaign. Shortly before, India had more cases than the rest of the world put together. The programme stopped trying to vaccinate the entire population and set about looking for patients instead: more than a hundred thousand trackers going house by house, village by village, one week a month, carrying a sheet of photographs of the rash to ask people whether they had seen anything like it. Teachers, barbers and market traders were paid to report. Searches were mounted in railway stations and at pilgrimages.

Geneva, 1 January 1967. The Intensified Eradication Programme begins, directed from the headquarters of the World Health Organization by the American epidemiologist Donald Henderson. The enemy's figures: between ten and fifteen million cases a year, some two million deaths, thirty-one countries where smallpox was endemic. The team's figures: an international budget of about two and a half million dollars a year. The Assembly had voted to eradicate smallpox eight years earlier, in 1959, and in those eight years almost nothing had happened.

Two things changed that absurd ratio between the problem and the money.

The first is an object. In 1965, a year before Foege reached Ogoja, Benjamin Rubin, a microbiologist at Wyeth Laboratories, flattened the eye of a sewing-machine needle and cut a fork-shaped notch into it. Held by capillary action between the two prongs was exactly one dose of vaccine. With that bifurcated needle a vaccinator used a quarter of the vaccine the previous method required, learned to use it in a quarter of an hour and sterilised it by boiling. Wyeth waived its royalties on it for as long as the campaign lasted.

The second explains why smallpox could be wiped out and almost nothing else has been. Smallpox has no animal reservoir: it does not hide in cows or mosquitoes waiting to come back. It has no healthy carriers: whoever has it shows it, and shows it on the skin, from the doorway, with no laboratory test. It takes about two weeks to pass from one patient to the next, which is an eternity for anyone chasing it. It leaves lifelong immunity. And it can be vaccinated against after infection: if you give someone the jab within three or four days of having been with a patient, the disease either never declares itself or arrives very mildly. There is a window in which to get there before the virus does.

And it was won with a fraction of the vaccine the manual demanded, because the whole method was improvised by a doctor who had run out of doses.

Ogoja, Nigeria, December 1966. A month before the programme started, William Foege, a thirty-year-old doctor posted to a mission hospital in eastern Nigeria, runs into an outbreak and the usual problem: the vaccine he has been sent is nowhere near enough to vaccinate the whole province. The manual of the day called for vaccinating eight out of ten inhabitants of a territory in order to stop the disease.

Foege did the sum the other way round. If the virus only travels from a visible patient to the people who live with him, there is no need to protect the whole province: what is needed is to surround each patient. He asked the missions and dispensaries in the area by radio to tell him which villages had the rash, marked those villages on a map and sent his vaccinators to give the jab only to the families, neighbours and visitors of each case. A ring around each spark, instead of soaking the whole forest.

Smallpox disappeared from eastern Nigeria with a fraction of the doses the manual called for and with a vaccination coverage that, on paper, was insufficient. Henderson ended up adopting the method for the entire programme, under a less elegant name: surveillance and containment. That Christmas, though, all there was was one epidemiologist with a few thousand doses and a province of hundreds of thousands of inhabitants, spending them entirely on the houses that surrounded the patients someone had marked for him on a map.