It is not hard even for the poorest and least educated people to remember to take one when the sun comes up and a second when the sun goes down.

Esther is one of the least fortunate children on earth, but one of the luckiest of those, although she can't know it.

At first glance she looks to be two or three years old, but her eyes are too large, her head is too big. She is placed on what seems like an enormous chair at the consultation table. She sits in silence, her wasted arms and legs hanging down. She is five years old, and although she has not been tested, the clinical officer has no doubt she has Aids.

"The child is always sick and is not growing or playing with other children," says Wangari Wanguru, the clinical officer. Esther weighs only 8.5kg (18lb 12oz). "It's not just malnutrition. A child with malnutrition would be playing. You see this apathy."

Esther was born in the Mathare slum on the outskirts of Nairobi, Kenya, where single men and women hoping for work in the capital end up making a sort of life in one-room metal-roofed shacks with no water, electricity or sanitation. The alleys teem with their small children.

Esther's mother died not long after she was born, probably of Aids. So did her sister. Her father now has another wife and does not want to look after a sickly infant. She is left with neighbours.

Her luck - against all the odds - is to live within a short walk of a concrete complex with blue gates and a corrugated roof of the same colour. It may not look special, but it spells hope for thousands of people who were born to nothing and now face imminent sickness and early death. The Blue House is one of the first medical centres in Africa to offer free treatment for those with HIV/Aids.

Today, on World Aids Day, a plan to roll out the sort of care offered at the Blue House throughout catastrophically hit sub-Saharan Africa and other poor countries is being launched by the World Health Organisation in Nairobi. Building on the work done by Médecins Sans Frontières, who run the Blue House, and other voluntary organisations, it plans radically simplified testing, diagnosis and drug treatment by specially trained healthcare workers - not doctors - in a bid to get three million in the developing world on treatment by 2005.

At the moment only 50,000 people with HIV/Aids in sub-Saharan Africa are on the drugs that can keep them alive. Esther's second stroke of good fortune is to have caught the attention of a motherly woman who lives nearby and who is one of them.

Benta could not miss the contrast between her own healthy four-year-old and the listless older girl half her size. Nobody in Mathare slum would doubt what was wrong with Esther, but they do not want to talk about it. Benta is different. She has brought the sick child to the Blue House because she knows the staff work miracles. Benta, who is HIV positive and nearly died last year from the tuberculosis that took over her body as the virus stripped out her immune system, has had her own life given back to her.

Médecins Sans Frontières has pioneered free drug treatment for HIV in the most destitute parts of Africa, something many in the US and Europe said would be impossible. The drugs were too expensive and too difficult to take, the diagnosis needed hi-tech tests and patients would have to be monitored long-term by doctors, they said.

At the Blue House, at a second clinic in rural Homa Bay near Lake Victoria, and in similar devastated parts of sub-Saharan Africa, MSF has patiently demolished every argument.

There is one fully qualified Kenyan doctor at the clinic, while the rest are nurses and clinical officers whose training puts them somewhere between the two. The Blue House treats tuberculosis and HIV. Most of those who arrive with TB will be HIV positive - in Homa Bay it is more than 80%. While the TB is being treated with a course of powerful antibiotics, patients will be advised to have a test for HIV. It is voluntary and there is counselling before and after.

A clinical officer uses a checklist of symptoms to "stage" those who are HIV positive. When Benta arrived, she was in stage four - the last. Those with TB are always stage three or four. They are underweight and their immune systems have been so badly depleted by the virus that they are prey to serious, life-threatening infections.

There still have to be blood tests. MSF does a CD4 count to establish the strength of the immune system, a liver function test and a full blood count, but the machines they use are relatively simple and, apart from the CD4 machine, most district hospitals would have them.

If the patient is stage three or four and shows a commitment to treatment by turning up for regular visits, they will be put on antiretroviral drugs (ARVs) which keep the level of virus low in the body. MSF buys the simplest, WHO-approved, most affordable drugs - a cocktail of three, taken in two pills.

It is not hard even for the poorest and least educated people to remember to take one when the sun comes up and a second when the sun goes down.

In Kenya, most of these drugs are under patent to the giant pharmaceutical companies and cost $767 (Ã?â?¦Ã?­445) per patient a year. MSF has to ask for a government licence each time it imports cheap generic copies from Ranbaxy in India, which cost $292 per patient a year. If those drugs fail to check the virus, patients must be put on a "second line", which includes patented drugs and costs $1,594, a steep bill for an African government. MSF says prices must come down further.

Down in Homa Bay, a remote rural settlement on Lake Victoria where one in three has HIV, MSF's clinic is part of the district hospital.

The wards tell a desperate story, overflowing with patients suffering from infections typical of HIV/Aids. They are two to a bed and sometimes there will be a third sleeping on the floor underneath. Most are on drips, many containing drugs to treat cryptococcal meningitis, which affects the lining of the brain. One woman, brought in by mission sisters, is writhing and shouting and has to be held still while she is given a tranquillising jab so that she can be examined. Only half will survive the meningitis. Others cannot eat and hardly speak because of oral thrush.

MSF has put doctors, clinical officers, nurses and other staff into the hospital to help with the Aids crisis. Those who recover on the wards are advised to have an HIV test. Because there is free treatment at the MSF clinic, the numbers coming for testing have soared from 1,600 in 2001 to 2,600 last year.

"It is one of the challenges we are facing in this programme," says Saleban Omar, field coordinator. "We are overwhelmed by the number of patients coming and looking for care now that ARV drugs are available. It is the only project free of charge in western Kenya and the people are very poor."

The Kenyan government recently put together a national Aids plan to treat 30,000 to 50,000 people within two years in 30 hospital centres. The very poor are expected to get drugs free, but others may have to pay half the cost.

Treatment would bring down hospital admissions by 60%, the government believes, yet in spite of the human misery in Homa Bay district hospital, the attitude of the official in charge of development in the district, Peter Ochiel, towards expanding on the work of MSF is defeatist. "At district level it is not very possible for us," he says.

Back at the Blue House, Helle Poulson, an Australian nurse, returns in triumph. She has been at Kenyatta hospital, searching for Julia, a 12-year-old girl with HIV who has been brought to the clinic several times by her older brother. The child had failed to turn up while he was away working and Helle was frantic. In her sickness, Julia was a burden to the family and other members had wanted to dump her in the middle of the city.

But Helle has found her, on a ward in the hospital, and relief shines out of her face. "We think we could save this one."

The tide of death that Aids has brought to the continent has left many numbed and hopeless. A couple of years ago, a child like Esther would have quickly died. Now she has every chance of surviving and becoming healthy enough to go to school and one day get a job. She may never escape the Mathare slum, but some do. She could one day be a doctor saving lives herself.