Before I began this piece, I did a little impromptu research outside my local supermarket. I stopped 20 people and asked them to say the first thing that came into their head when I mentioned the Congo. Of my sample: 11 remembered the slogan of a popular Eighties fruit drink; 3 said it was in Africa; 2 said it was a river, 2 thought there might be a war there; one looked confused and one said; The what? My friends and family hardly scored better. When I told them, last year, that I was going to Congo-Brazzaville, reactions ranged from; That'll be nice, to You're completely mad, to Why?

Well, that one's easy. Years ago, I read an angry article by AA Gill on the subject of sleeping sickness. I've never forgotten it. It got under my skin. That's why I decided to give MSF the proceeds of my cookbook, and promised that if ever I could raise awareness for any of their projects, I would. That's why I'm here now, at the airport, with my rucksack, mosquito net, malaria pills, enough survival equipment to make Ray Mears pale with envy, and a ticket in my pocket for Congo-Brazzaville.

First, some facts. The Congo is an African country about the size of Germany. Most of the population (just under 3 million) live in the capital, Brazzaville, or its coastal counterpart, Pointe-Noire. Average age is about 20; average life expectancy, just under 50.

The population speaks mostly French, Lingala (the lingua franca of the region) and a number of local languages. Its president is Sassou Nguesso, who came to power in 1997, having toppled the elected president, Patrick Lissouba. Much of the interim time has seen conflict between Government militia and a number of rebel groups, with whom an uneasy peace agreement has now been reached.

I awake early on my first day to the sound of drumming and clapping from the nearby church. Brazzaville is a crossroads between worlds; bombed-out buildings stand alongside elegant glass towers; suited businessmen mix with ragged street children, expensive European cars alongside battered and psychedelic taxis.

After breakfast, we drive out of the city towards Ngangalingolo, on the outskirts of Brazzaville. MSF has an emergency centre in this area; since October 2002, thousands of displaced and destitute people from Brazzaville and the Pool region have ended up here, living in crowded camps and subsisting on whatever they can forage.

The Pool region, once a prosperous area, rich in cattle and agriculture, has been devastated by conflict between the government and the Ninja rebels, has endured a vicious series of "scorched-earth" reprisals from the military and is still in the grip of various bandits, rebels and military groups.

Children, women and the elderly have suffered most from a war where civilians were targeted as often as the enemy; in some cases MSF's food distribution was all that kept these people from starving. It is pitifully little; even now, almost a year later and with the crisis approaching its end, there is anxiety in the faces of the mothers in the food line as they await the twice-monthly distribution of dry food (rice, soya and cornmeal).

Paul, MSF's chef de mission, speaks to the leaders of the camp, who have many concerns. There are no blankets, no tools, no mosquito nets; 57 babies have been born here since the camp was set up, and the government hospital is demanding that pregnant women pay for antenatal care, which is (supposedly) provided free of charge. Paul tries to help, to reassure, to explain the role of MSF - all their medical treatments are free, but the government hospitals are outside their control, and MSF is unable to provide the material things they so badly need. It seems a thankless task, and yet Paul remains cheerful and patient, although I sense he has had this conversation many, many times before.

Julienne is in her thirties, with a worn, still-beautiful face, and has clearly dressed in her very best clothes to come to the hospital. I'm a widow, with four children. My husband was killed during the war. Last October, soldiers came to our village. They had machine guns and machetes and torches. They knocked on our doors in the middle of the night, demanding money. When we told them we didn't have any, they took some of the little girls away and raped them. I asked God to help us, but He didn't.

Julienne has not returned to her village since. For two months she and the other villagers hid in the forest, foraging for food as the helicopters passed overhead. At last, they decided to risk the dangerous three-day journey to the camp. It is a bleak and filthy place; families are crammed together, sleeping on mats on the floor. Mountains of litter rise up out of the dust. Old women squat around cooking-fires; an old man sits making baskets in a corner.

It's hard to believe that this is progress.

I ask Julienne if she will go home, now the crisis is over. Home to what? There's nothing left. Soldiers burnt the village - took everything, even the bricks. There's peace now, so they tell us. But that's what they told us last time.

>Tongo Today, we begin the long journey to Mossaka, where MSF have their new sleeping sickness programme. It will take two days; one by road, one by river. We are soon out of Brazzaville; the tarmac road soon giving way to dirt. We see huge, overloaded trucks crawling along in clouds of dust, while the passengers - forty, fifty at a time - cling to every available surface. People wave and call: mendele, mendele. There are sandtraps on the road, and roadblocks every few miles, some military, some not.

People are expected to pay at these roadblocks, explains MSF's medical coordinator, Sarwat, but it is MSF's policy never to pay, and most of the time people respect it. Occasionally, though, there is trouble.

Not long ago, in Voula, Sarwat was kidnapped at gunpoint by a group of Ninjas when she refused to let them commandeer her vehicle. They took it anyway, and dumped it - and her - unharmed once their business was done. Sarwat - who weighs forty kilos soaking wet, and has the spirit of a tiger - speaks of the incident with astonishing good cheer; these things happen, she says, and no harm was done. At first, I hold my breath as we approach the roadblocks, but as the day goes by I hardly notice them.

At dusk we arrive at the village of Tongo. From here we must travel the remaining distance to Mossaka - about 45 km - by pirogue: a hollowed-out tree with an outboard motor tacked onto the end; the trip will take at least six hours. Night falls; we have time to set up our mattresses on the floor of a mud-brick hut intended for the village schoolteachers, when term starts next week. There is no electricity here; we eat outside, in the dark, to the sound of drums and cicadas, as the fireflies blink on and off in the bushes like the world's longest string of fairy lights.

We leave at six the next morning. No-one has slept well; we wash furtively in bottled water beneath the curious gaze of the village urchins, to whom we are impossibly exotic. We take the pirogue up the Alima river, then along a series of narrow, rushbordered canals into the Congo.

This is what I have been most looking forward to; the sound of the engine, the rush of the brown water, the fishermen in their tiny pirogues skating along the banks. When we stop to refuel, the orchestra begins; monkeys, frogs, birds and cicadas. The river is several miles wide at this point; no-one knows how deep.

Mosquito Hotel.

The house in Mossaka is pretty luxurious by local standards. There is a powerful smell of bats - we will hear them squeaking and skittering throughout the night, but the floor is tiled; there are two bathrooms (but no running water) and an electricity generator. The people here are amazingly friendly and welcoming.

There are half a dozen expats, all from different countries, all desperate for gossip and news of the world back home. There is a strong sense of community. I feel that I could get drawn into this very rapidly; I begin to see why some volunteers find it hard to leave.

Today we visit the hospital where the sleeping sickness programme is already under way. MSF's plan is to screen every member of the population - an awesome task, given the difficulty of access and the epidemic nature of the disease. Mossaka is a good place to begin; there are 10,000 people living here on the river, mostly in mud-brick huts and without sanitation or medical supplies.

Proximity to the water brings its own health problems, and when the rainy season begins, there will be flooding. In a couple of weeks' time all roads will be cut off, and the MSF doctors will be paddling to the hospital in pirogues. Insects love it - the malaria-carrying mosquito, the tumbu fly and of course, the tsetse fly, which harbours the parasite - the trypanosome - that causes sleeping sickness.

It is a foul disease. Almost eradicated in the &#‘60s, it has returned on an epidemic scale, killing about 66,000 a year.

Ultimately fatal, it can be cured, although until recently the only available treatment was an antiquated arsenic-based compound, Malarsoprol - an intensely painful, dangerous - and sometimes deadly - cure. Until recently, no-one had funded any research to an alternative. Then it was discovered that DFMO, a drug originally developed to treat cancer, made the trypanosome infertile. The only problem was that no-one wanted to produce it after it was discovered not to work in cancer - sleeping sickness is not a lucrative disease. Finally, after a long campaign by MSF and the World Health organisation, the manufacturers of DFMO agreed to make a limited amount of the drug. We can see the results in Mossaka.

There is already a queue for screening when we arrive at the hospital. The MSF team have worked hard to educate people about the disease; to emphasize the risk to all; the lack of early symptoms; the importance of screening everyone. Superstition here is very strong; many illnesses are blamed on witchcraft.

A family group is waiting by the desk; a mother, a father and the three youngest of seven children. They have travelled two days by pirogue, the father tells me; so far, they are the only people from their village to have come. The father fell ill with late-stage sleeping sickness last November, he tells me. Lethargy, insomnia, psychotic interludes, blindness, paralysis - he was dying. His family brought him here, where Malarsoprol saved his life.

It was like fire, he says. The injections take a long time; sometimes the needle takes twenty minutes to empty into the vein. I was in hospital for two months, with treatments every other day. But when I came home, I could walk again. He is back today with his family for a follow-up screening. With luck, the disease has not resurfaced. It begins with a pinprick, a blood test. The mother takes it first; the children begin to cry, then to scream in fear at the sight of blood. The eldest bites his lip bravely; a skinny little boy trying hard to give a good example to his little brother and sister, who struggle and wail. We wait with them in the rest area, while the technicians inspect the samples.

Screening for sleeping sickness is not a straightforward one. It is a complex laboratory-based process, encompassing several stages, and is a challenge even in a modern hospital environment.

Here, the MSF doctors - recruited locally for the most part - will screen hundreds of people a day.

The family waits anxiously for the results of the test. The news is mixed; the father, mother and two youngest children have tested negative, but the five-year-old shows a positive result. More tests are needed; a physical examination to check for the presence of ganglia in the neck, then a sample of fluid from the ganglia. The little boy looks close to panicking; the nurse tells him to be brave; another blood test, this time from the vein, must be taken. If this too shows positive, the doctor will need to make a lumbar puncture, and tap fluid directly from the spine. It is a painful, frightening process; even as the boy waits for his result, we can see a young woman undergoing the operation in an adjacent room.

She weeps silently and continuously as the long needle drains, but stays completely still.

The five-year-old is a mass of twitches; his face is pinched; his eyes huge. I have some sweets in my pocket; I hand one to him as two nurses lead him into the next room. As he sees the needle he begins to scream; one nurse holds him firmly while the other tries to take the sample. He struggles so hard that the first needle misses its target; the second is on target but is almost shaken free by the boy's panic-stricken movements. He screams for his father, his mother, God, and finally his sweet, which has fallen to the floor.

It is a traumatic process, but the nurses only smile - after all, every positive test is a life saved. I begin to feel a tremendous admiration for the patience, humour and calm of the staff here.

Sucking his sweet, the little boy is laid to rest on a mat beside his father. Now they must wait again to find out whether the child is a Phase 1 sufferer, with the parasite in his bloodstream, or a phase 2, where the trypanosome has already migrated to the brain. Both are treatable; but Phase 1 can be treated on a relatively simple outpatient basis, while Phase 2 must be treated in hospital, with 3-hourly infusions of DFMO.

A cry in the night I am awoken at 2am by screams of anguish from outside the compound. Someone has died during the night. It often happens this way, says Rhian, the project co-ordinator; and the lamentations will continue for many nights, especially if the victim was a child. Child mortality is high; malaria is the greatest culprit, but even diarrhoea is a killer.

Europeans tend to assume that Africans take this in their stride; they get used to it, somehow, as if the death of an African child were somehow less important, less heartbreaking than that of one of ours. It isn't true; children are valued here more than anything else; and even their names - Désiré, Joyeux, Bien-Aimé, Je T'aime - reflect the joy they bring their parents.

After breakfast we return to the hospital to look at the wards. Each one contains a dozen people, mostly children.

We recognize the little boy from yesterday, huddled and asleep on his bed. He has been diagnosed as Phase 2, and must stay here for up to three weeks.

We set off once more for Tongo. The mobile clinic is large enough to take 20 people - doctors, nurses and aides - as well as the camping gear and all the laboratory and screening materials. MSF will try to screen, process and treat the entire population of the village before moving on upriver. It sounds impossible, but I am assured it can be done.

Gérard and the others have worked hard to spread the word about tomorrow's screening. The head man of the village, a Presidential type in gold-rimmed aviators, must be deferred to and consulted; it is clear that he feels slightly threatened by the operation, and it takes a good deal of diplomacy and tact to bring him round.

The team works steadily and with unflinching good humour as the day wears on. The heat grows; the smell of sweat and chemicals is almost overwhelming. And the people keep coming; old women with hip problems, children with malaria, old men who have gone blind.

By four o' clock, the line of people has cleared. No-one else will come now. Gérard, Basile and the others have screened the entire population of Tongo - 712 men, women and children - in a single day. It's a relief and a triumph.

When night falls, the aides light a bonfire. They should be exhausted; but instead, everyone seems infused with energy and excitement. Basile makes a drum out of an old water-tank and everyone improvises songs, chants and sketches in celebration of MSF, the good work that has been done in Tongo and their pride in what they have achieved.

It is touching; and I feel privileged to have been here, to have stood alongside these people and been a part of what they do. Their goal - mass screening and total eradication of sleeping sickness in the region - may sound impossible, but somehow I don't think that word has much of a place in their vocabulary. They are still singing and dancing when I go back to the hut. I get the feeling the celebrations have only just begun.

Joanne Harris is the author of four critically acclaimed novels: Coastliners, Five Quarters of the Orange, Blackberry Wine, and Chocolat, which was nominated for the Whitbread Award. She donated the profits from her latest cookbook, 'My French Kitchen', to MSF.