One child's salvation is a symbol of the stark ethical and economic choices facing doctors and scientists as they gather in Bangkok this weekend for the world's 15th International Aids Conference.

For only 200 people in Burma are getting the same treatment as Min Oo - even though at least 180,000 and perhaps as many as 450,000 others in the country are infected.

Only one international aid group has started to give Burmese patients the anti-retrovirals - the free drugs he is receiving - that can stop the disease in its tracks.

The cost is $1 (53p) a day. That would be too expensive for most of the victims and far beyond the finances of Burma's military rulers, who prefer to spend a large proportion of their budget on Russian warplanes and Chinese weapons.

Min Oo was dying when his mother, who is also carrying HIV, brought him to a clinic run by the Dutch section of Médecins Sans Frontières, the charity.

"He began the treatment on June 8, when he had tuberculosis, oral infections, malnutrition and diarrhoea," said a nurse, consulting his file. "As you can see, he has done very well."

Mother and child squatted on the floor of the clinic, a collection of bamboo huts perched above the monsoon mud in a shanty town called Hlaing Thar Yar on the outskirts of Rangoon, the child gazing mutely around him while his mother smiled and cooed in his ear.

Another little boy, Aung Thet Aung, 4, started the complex triple drug therapy on March 1, when tuberculosis was ravaging his body. Now he bounced around and wolfed his mid-morning snack under the watchful eye of his mother, who is also receiving the free drugs.

"We have had some very hard decisions in choosing which patients get the therapy," said Per Bjorkman, a Swedish specialist who treats them. "We have been besieged. Some people have walked for four or five hours to reach our clinics but the resources are limited."

It is only an hour by plane from the air-conditioned conference halls in Bangkok in which 15,000 delegates will gather today, but Hlaing Thar Yar, where sex is traded for survival amid the shanties, might as well be in another world.

Children such as Khin Myat Noo, 7, orphaned by Aids and now fighting the disease herself, are at the sharp end of the abstract debates about priorities on the conference agenda.

She sat on her cousin's lap while Bjorkman used his stethoscope to detect the treacherous traces of pneumonia remaining in her left lung. If she is to survive to adulthood, she will have to take the anti-retrovirals until researchers find a substitute drug or a cure.

Until recently, there was a consensus that money should go into education and prevention to save untold millions from getting Aids, rather than on trying to save those already fatally ill.

But the discovery of anti-retrovirals - whose cost and availability became a global political issue at the Aids conference in Durban four years ago - has changed the moral equation.

"Until we had the triple therapy there was little we could do to give hope," explained Bjorkman. "Now we can give hope, there are a whole new set of problems."

Here in the fetid shanties of Rangoon, those problems pose an acute dilemma for Bjorkman and his colleagues. How open-ended is their commitment to Min Oo, the two-year-old with the luminous eyes, and the rest of the lucky few? "Okay, there is a debate about prevention doing more good than treatment," acknowledged Frank Smithuis, a bluff Dutchman who heads Médecins Sans Frontières in Burma. "But if you're not careful you find it has paralysed you to the extent you do nothing."

Smithuis negotiated hard with the Burmese regime - which until recently refused to admit that the country had an Aids crisis - and won approval for the trial, which aims to treat 500 people by next year.

After three years, he hopes to hand over their care to the government system. By then Burma should be reaping the benefits of a massive new United Nations programme that should transform the children's prospects.

Britain, the programme's principal provider of funds, has set aside its aversion to the military junta to commit £10m; and the BBC World Service is broadcasting a weekly soap opera to Burma in which rural doctors fight superstition and ignorance about HIV.

Such intervention is still politically sensitive for the regime. Large-scale UN programmes also raise questions about the bureaucracy of Aids, which some doctors in the field believe has spread as remorselessly as the virus itself.

The organisers of the Bangkok conference are not afraid of controversies: free condoms will be handed out to delegates and there are plans for a go-go dancing show by professional sex workers.

But the event itself has attracted criticism. It will cost more than £7.7m to stage - a breathtaking figure when contrasted to the 53p a day needed to keep these Burmese children alive.