When Philimon Banda fell ill last year in his small village on the edge of Lake Mweru in northern Zambia, he went to 15 local witchdoctors who all told him he had been possessed. One said he had a snake in his body drinking his blood, another that he had been inhabited by a ghost, a third that he had been bewitched by jealous neighbours.

They took his money, washed his evil spirits away, exorcised him and gave him roots and powders. But he got progressively weaker until, he said, by early this year he could not even walk.

Five months ago Mr Banda was tested positive for HIV by Médecins sans Frontières and put on antiretroviral drugs, which have saved his life. He is now strong enough to work and angry with the healers. "It was very wrong of them to promise they could cure me," he said.

But the power of traditional healers in northern Zambia, where up to 25% of the population is HIV-infected, is enormous. People go to them first and treatment can be fatally delayed - something that bothers the witchdoctors, too.

"It's a very old disease which we call umukola," said Chishimba Kilimanjaro, a prominent Congolese witchdoctor who has been exorcising spirits to cure people of the disease for years near Nchelenge. "It can be caused by people bewitching each other, or be passed from a dead man's wife to his family. It is easy to diagnose and treat but everything depends on how long the ghost has been in the body. The only people who cannot be helped are ones who - like Mr Banda - only come when the illness is far advanced," he says.

He attracts people from all over Zambia and Congo to his circle of 15 straw huts by the lake and claims to be able to cure them if they come early enough. He will not say if he saw Mr Banda.

Western and traditional medicine in Zambia are culturally miles apart, but both camps agree that attitudes to sex, health, wealth and death have helped HIV to spread. Changes in cultural practices are needed and doctors of different disciplines must cooperate. Dr Kilimanjaro, like many other healers, now sends people with advanced Aids to MSF.

"HIV is largely spread round here because of polygamy," said Moses Banda, an HIV-infected teacher. "Our culture accepts that a man can have five or more wives. That means that the disease can be spread very far. Then there's the widespread practice of dry sex. Women apply traditional plants to their private parts which makes it more stimulating for the man. But this can lead to abrasions and tears," he says.

Even riskier, say many people, is the southern African practice of "sexual cleansing", to get rid of the ghost of a dead spouse. In Zambia, it usually means that the wife of a dead man must have sex with his brother, sending the infection through communities. It has been officially banned by most local chiefs and the Zambian government. "Things change slowly, but it is still very common here," said Moses Banda.

"There are so many cultural misconceptions," said Fancy, a counsellor at MSF's Bum Bwesu information centre. "Some people think there is no such thing as HIV, others that you become infected as a result of sleeping with a widow. They believe it is a ghost that is haunting them ... People are afraid to be tested but the most afraid are the middle classes. They do not want to be tested and are very afraid of being shamed. The best thing we can do is give people information. Happily, there is much less stigma now. Just four years ago people thought that they could catch HIV/Aids from being in the MSF car. People are more open about their status now"

Elizabeth Senkwewmenya, the district coordinator of the Zambian Peoples Living With Aids group, is HIV positive. She visits communities, encouraging and educating groups of infected people. She said: "Men are frightened. Women are vulnerable. The poor are desperate. They really do not want to know, even if it means dying. Some men are polygamists. A lot of poor women are so desperate that they go with people who have money. People fear going to clinics, fear having fingers pointed at them, they think that they will not feel the same if they put a condom on. Some men believe that if you sleep with a young woman you will be cured. Others that if you eat African potatoes you will get better. Imagine.

"People are dying just because of misconceptions and lack of information. As they learn, so they lose their fear. We are beginning to change how people think. It's much more positive now."