Johann Hari: The parasite that reveals good news from Africa

And now for the great news – from Africa. Yes, I know that seems like a perverse opener, with Robert Mugabe perpetuating his oozing Alzheimocracy, a looming famine in Ethiopia, and international peacekeepers failing to prevent genocidal massacres in Darfur. The cynics who jeer that Africa is a black hole for help feel they have the wind of no change at their back. But some time next year – or soon after – a beautiful moment in the history of humanity will come to pass on the Western shores of Africa. An excruciatingly painful disease that has stalked humans for millennia will end – forever.

The story of how this came to pass begins just 20 years ago, in a tiny village in Ghana. The former US President Jimmy Carter stumbled across a crying woman who appeared to be cradling a baby to her right breast. He stepped forward to talk to her – but he reeled back when he realised a 3ft-long worm was inching its way out of her nipple, at the centre of an engorged, purpling breast. It was one of 11 guinea worms taking a month or more to crawl out of the young woman's body that summer. One was burrowing out from her vagina. The woman couldn't speak; she could only howl.

She was living through a guinea worm infestation. One survivor, Hyacinth Igelle, says: "The pain is like if you stab somebody. It is like fire. You feel it even in your heart." After seeing some victims, the journalist Nicholas Kristof called it "torture by worms". The worm's head causes a blister that often develops deadly tetanus; if the victims survive, they can starve because they have not been able to farm their fields for months. Many scholars now believe that when the Old Testament Israelites were afflicted by "fiery serpents" in their flesh, they were meeting this worm for the first time.

When Jimmy Carter first encountered the disease, some 3.5 million people were riddled with guinea worm. Tens of millions of people had endured it from Europe to Asia; it was regarded as an intractable, eternal problem. The idea of eradicating it was mocked as "utopian". But today, the number has been slashed by more than 99 per cent. Fewer than 10,000 people, in a few remaining pockets of Ghana and Sudan, still suffer – and soon, there will be no one at all.

This achievement is all the more startling when you realise there is no vaccination or cure for the disease. Guinea worm eggs are carried on the backs of a tiny water-flea, and glugged down by humans with their drinking water. The eggs hatch in your abdomen, growing over a year to 3ft long – and then they begin to dig their way out. They can choose any point of your body to emerge from: your eyeball, your penis, your feet, destroying as they go. As they do, they spew millions more eggs into any water they come into contact with. Once the worm is within you, the only help doctors can offer is to wait until it bursts out and wrap the worm's head round a stick to try to very gently tug it out a little faster.

But you can stop people contracting the parasite in the first place – and Carter has, on a massive scale. The practices are startlingly simple: the distribution of egg-catching water filters that cost around 60 pence each, and mass education about why they matter. But it took a vast effort to get them in place, including brokering a "guinea worm ceasefire" to the Sudanese civil war that allowed aid workers free access. So Carter raised $225m (£123m) from governments and private donors, and used it to drive the worms off the earth, one village at a time. At 84, he is determined to outlive the last of these little parasites.

This Carter-led programme is sending guinea worm to the mourner-free graveyard of eradicated diseases, along with smallpox and (soon) polio. But it doesn't end there. In a cynicism-drugged age, it is a reminder of what we can do, if we have the determination.

Our governments are very good at building weapons of mass destruction – but for a fraction of the cash they could unleash weapons of mass salvation, eradicating disease after disease. This programme should flush away the glib cynicism about aid to Africa along with the worm-eggs. It proves money from outside, if used intelligently, can massively improve the lives of ordinary Africans. Indeed, it can achieve goals that seemed at the start like utopian fantasies; it can reverse the curses of millennia.

One day soon, the last guinea worm will burrow out of its last victim. I want to take all those shallow, callow contrarians who say aid to Africa is worthless to witness that moment – and see if they still shrug quite so casually.

j.hari@independent.co.uk