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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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When species extinction is a good thing

Will Jimmy Carter exterminate Guinea worm soon?

It's not a race, exactly, but there's an intriguing uncertainty about whether a former U.S. president or a software magnate will cause the next deliberate extinction of a species in the wild. Will Jimmy Carter eradicate Guinea worm before Bill Gates eradicates polio?

It is more than a third of a century since a human disease was extinguished. The last case of smallpox was in 1977, and in those days health experts expected other diseases to follow smallpox quickly into oblivion. Polio has repeatedly disappointed campaigners by hanging on, though it now affects less than 1% as many people as at its peak in the 1950s.

The generosity of Bill Gates has done much to speed the decline of polio, and he and most experts now see its end within six years at most. India, 10 years ago the worst-affected country, has been polio-free since 2011, and only three countries still host the virus: Pakistan, Afghanistan and especially Nigeria. Though the murder of nine polio vaccinators in Pakistan by Islamists in December was a tragic setback, last year there were just 222 new polio cases world-wide.

As Mr. Gates recounted in his 2013 annual letter from the Gates Foundation, the reason for his optimism is that a new approach is bearing fruit, especially in northern Nigeria. Volunteers on foot (but guided by GPS and satellite imagery) map unrecorded villages and houses to identify gaps in vaccination programs.

The Guinea worm, a disease that the Carter Center has relentlessly pursued, will probably edge out polio to the disease extinction line. In 1986, more than 3.5 million Africans and Asians were afflicted with Guinea worm, or dracunculiasis; in 2012, just 542 caught the parasite.

The larvae of this nematode worm live inside freshwater copepods, or "water fleas." When the copepods are ingested in drinking water, the worms burrow through the stomach wall into the body cavity and mate. The females, which can reach 3 feet in length, then drill their way down the inside of the victim's legs over a year before erupting painfully from a burning blister on the foot. The victim is tempted to immerse the blister in water to cool it, which allows the worm to release its larvae to seek copepods. The only cure is to pull the worm out over many weeks, inch by inch, winding it round a stick as it emerges. There is no vaccine.

Filtering water to prevent the ingestion of water fleas and making sure infected people do not enter water are the best means of prevention. Guinea worm was first targeted for eradication before polio, and it, too, has been disappointingly stubborn. But last year the number of cases halved from the year before, meaning that there are fewer guinea worms left in the world than black rhinos. The handful of cases in Chad (10), Mali (7) and Ethiopia (4) are expected to dwindle to nothing this year, but there were 521 cases in South Sudan (mostly in just one county), where eradication might take one or two more years of hard work, urged on by Mr. Carter and backed by money from the Gates Foundation, the British government and other donors. Guinea worm would be the first animal to be deliberately driven extinct.

Supposing these two welcome eradications do happen this decade, what parasites go next? Don Hopkins of the Carter Center says lymphatic filariasis, another worm carried by mosquitoes, could be gone by 2020. Onchocerciasis, or river blindness, carried by black flies, is almost gone from the Americas but will take longer to eradicate in Africa.

The first bacterium to be driven extinct could be yaws, an infection of children related to the organism that causes syphilis, which disfigures many people, especially in Ghana, Ivory Coast, Papua New Guinea and the Solomon Islands. Easily treated now with a single dose of azithromycin, an antibiotic, yaws should be gone by 2020.