The World Health Organization has recommended a new treatment for HIV-positive children which researchers say can dramatically cut death rates.
Co-trimoxazole costs less than ten 10 per child a day
The drug - a common antibiotic called co-trimoxazole - costs less than 10 cents per person a day.
A trial on children in Zambia suggests it can nearly halve mortality rates for infections such as pneumonia and tuberculosis, often caused by HIV.
Every day about 1,300 children die from HIV/Aids illnesses across the world.
The Zambia trial was carried out by doctors from the UK's Medical Research Council.
The study ended early when they realised how effective of the treatment was.
After about 19 months, a quarter of the children who had been taking co-trimoxazole had died, compared with more than 40% of the children who had been given a placebo.
The doctors, along with the WHO and Unicef, are now calling for the antibiotic to be given to all HIV-positive children in developing countries.
'Treat all children'
Previously, experts had doubted whether it was necessary to give HIV-positive children the drug if they showed no signs of disease.
All of the children in the trial are now on preventative co-trimoxazole and those who needed antiretroviral therapy are starting it.
Dr Dianna Gibb, who led the research team, said: "The results of this trial should provide an impetus to provide clinical care with co-trimoxazole prophylaxis and nutritional support, irrespective of levels of resistance to this drug."
The UK International Development Secretary Hilary Benn called the findings "a breakthrough in medical research which can help to save children's lives all over the world".
A WHO spokeswoman said: "Until you can confirm the child's HIV status they should be taking co-trimoxazole.
"That would apply not just in Africa, but certainly in other countries where children do not necessarily have access to antiretroviral drugs."
She said co-trimoxazole had the advantage of being cheap and readily available, where as antiretroviral drugs are more expensive.