Thirty years ago, Lake Mweru in the far north of Zambia had so many fish it was said a man could catch a tonne a day with just two small nets. But few people lived near the great inland sea then, so the fish were barely taken. But increasing poverty, conflict on the Democratic Republic of Congo side of the lake and the closure of Zambian copper mines have lured tens of thousands of young men to the lake to start fishing.

Stocks have plummeted alarmingly and there's a saying in the villages that these days it is is easier to catch HIV/Aids on Lake Mweru than fish.

"This is a crossroads for Aids. The illness here is terrible," said Mrs Muhone, from the copper belt region of Zambia. She spends several months a year here, buying salt fish from the temporary fishing camps which line the lakeside then selling them back in the towns.

"People come here from everywhere - Zimbabwe, South Africa, Nigeria, Malawi, Congo," she said. "When they meet, well, you know what nature does. Wherever there are people there are temporary marriages. Men cannot stay without their wives for very long and women may be desperate, so they sell themselves. HIV starts here and moves on. It is very, very common here."

The lake and the camps are the frontline of the disease in Africa. Towns and villages have sprung up from nowhere and a tar road to the south of Zambia makes trade easy. Poor women have flocked in to become sex workers, the borders are porous and there is money around.

The statistics make terrible reading. Médecins sans Frontières came to Nchelenge, the largest lakeside town, in 2001 and has been testing 400 people a month. One in four people aged 15-49 are HIV-positive. Of these, perhaps 1,500 are at an advanced stage of the disease and need immediate anti-retroviral drug treatment. Life expectancy has dipped below 40 years and there are hundreds of orphans. Almost every family has at least one person with the virus.

MSF estimates that as many as 10,000 people in the villages and towns that depend on the lake are now infected. They have put 350 people on anti-retrovirals so far, a figure expected to double in a year as the drugs become cheaper and more available.

Gertrude, a fish trader from the south of Zambia, thought she had caught malaria after coming to the lake - but did not recover. "I thought I was OK but I was just not getting better. I could not work." She tested positive for HIV and is now on anti-retrovirals, hoping to get enough strength to return home.

Doreen is one of at least 80 sex workers in Nchelenge and perhaps 400 in the wider communities. She came to earn money, but finds it hard to be assertive. "I am 20, from Congo, but most of the girls are just children. I take one or two men a night, but some have four or five. Some men really do not want to wear condoms. It's hard to get them to. Others offer to pay more for not wearing one. Some tell lies and say they have worn one with me but haven't. Some offer to marry me but they disappear in the morning. We talk about our HIV status. I tell them I don't know it."

But she does. Four months ago she tested negative for the virus and says she does not want to take risks again. But she admits she still has unprotected sex with her regular boyfriend, a married man who says he has not been tested. "I thought I was infected. I think I have learned," she says.

Alex Kunda, who supervises a team of MSF counsellors, said the young men here were at serious risk. "Their mentality is to sleep with as many women as possible. Some see it as a sign of manhood. There are many myths about condoms. They say that the lubricants give you a stomach ache, that it is dangerous to 'bathe with a raincoat on' [have sex with a condom]. They abuse them. They make footballs out of them. The girls wear them as bracelets. Some businesses pay men in condoms now."

Veronica Muzinga, who employs two fishermen and exports the fish across the lake to towns in Congo, said the disease was rampant. "It's because people from all over Africa go back and forth across the lake so much. There are many young men. They may have several temporary wives, they give the disease to the women and together they spread it everywhere. I have been coming here for 11 years and it's certainly worse."

Mr Kunda said education was crucial. "There have been some good changes in five years. People are more aware. Everyone knows of HIV/Aids now. Now some of the 18- to 24-year-olds come for condoms. They are getting information but we are never sure they are using condoms. We leave them in clubs, guesthouses, in public squares. We have counselled thousands of people. It's making a difference but it's still little. There's so much more to be done."