Eunice dos Santos is a forbidding character. From behind her big desk, she dispenses the drugs and advice that save lives by reducing the likelihood that an HIV-positive mother will pass the virus to her child. Her stern manner softens as she scoops up Keith and sits the plump 11-month-old on her lap. If he tests negative for HIV at 18 months, it will be in part due to Mrs dos Santos and the relationship she has built up with his mother.

Keith's mother, Josina Muchanga, 30, has three other children and one on the way. After she was diagnosed as HIV-positive, she was sent to Mrs dos Santos's clinic in Maputo, Mozambique. Without the sophisticated equipment available in the west, the service given to new mothers is basic. At 28 weeks, Ms Muchanga received a tablet of the antiretroviral drug neviropene to take as she went into labour. Shortly after Keith was born, he was also given a dose.

The clinic has been able to prevent 80% of infants contracting HIV with these two tablets, and the success rate would increase if babies were born by caesarean. But costs and the risk of other infections are too high. And Ms Muchanga is one of only a tiny minority of Mozambicans to be given the drug. In 2003, only 3% of women asked for and got the results of an HIV test.

The second - and more difficult - piece of advice Mrs dos Santos gives pregnant women is to breastfeed for six months and then wean quickly in a few days. Breastfeeding increases the risk of transmission, and in the west HIV-positive mothers use artificial milk. Until recently, Mrs dos Santos distributed milk powder at the cost of £9 a month per baby but the government stopped it, saying it was too expensive.

The country's priority is to expand the programme to prevent mother-to-child transmission as fast as possible - but that would overwhelm the government's resources. At the moment, 35,000 babies are diagnosed with HIV every year in Mozambique - perhaps a third of those are infected postnatally.

"I'm very worried that the switch from artificial milk will reduce our success rate," said Mrs dos Santos. "My major concern is that mothers still buy milk powder but they don't ... boil the water or they don't have the clean materials for preparing bottles."

But the government's decision is supported by Médecins sans Frontières, which helped pioneer the programme. There is a difficult balance to be struck between the risk of an HIV-positive mother transmitting the virus to her child through breastfeeding and the dangers presented by unsafe water and artificial milk. Studies have shown that, in Africa, a baby fed on artificial milk in the first two months of life is six times more likely to die from infectious diseases than a breastfed baby.

Those risks are evident in Arminda Gabriel's tiny shack in Maputo. She cooks the family's meagre meals on a tiny stove balanced on the hard-packed earth outside their shack, foetid water sits in an empty bucket and flies buzz over the small children. She is HIV-positive and on antiretroviral treatment. Her three-year-old son, Juan, is also HIV-positive, but too ill with malaria to start taking the drugs. It is too early to tell whether two-month-old Carlos, sucking at his mother's breast, is positive - Mozambique cannot afford the early tests for HIV used in the west.

"I'm trying to prevent my baby getting HIV, that's why I go to the clinic for the consultation," said Ms Gabriel. "I will feed the baby for six months and then switch to maize porridge. I can get carbon to boil the water." But the family have to buy their water from the nearby standpipe and buy the carbon for fuel, and they have only Ms Gabriel's tiny income from washing clothes - her husband is too ill to work.

And she faces another difficult task: weaning. It is standard practice in much of Africa to breastfeed for two years or longer, but mixing breastmilk with other foods has been shown to increase the risk of HIV transmission. Even maize porridge can make slight abrasions on an infant's throat and mouth. Normally these are harmless, but if the mother's milk contains HIV they increase the likelihood the child will contract it.

The World Health Organisation has recommended that babies be weaned in two or three days, but as any mother knows this is very difficult to do, especially if you cannot afford artificial milk. There is a strong temptation to quieten a baby's cries with cheap breastmilk.

For Ms Gabriel, the importance of sticking to the advice is clear: she only has to look at Carlos's older brother, Juan, who is about the size of a one-year-old with limbs as thin and fragile as sticks. His eyes seem huge in his prematurely wizened face.