Giving Africa's HIV-positive children a cheap antibiotic could nearly halve the death rate, research shows.
Co-trimoxazole costs less than ten cents per person a day
The Medical Research Council trial in Zambia was stopped early when it became obvious how effective daily co-trimoxazole treatment was.
The World Health Organization and Unicef are altering their drug advice in line with the Lancet study.
Every day as many as 1,300 children die from HIV and Aids-related illnesses across the world.
Dr Dianna Gibb and colleagues studied 541 children with HIV symptoms aged between one and 14 living in Zambia, where drug resistance to common antibiotics is widespread.
Despite this, the children who were given daily co-trimoxazole fared better than those who were given a dummy drug. Treating somebody with the antibiotic costs just a few pence a day.
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 the dummy drug.
'Treat all children'
Previously, there had been questions over whether it was necessary to give HIV-positive children co-trimoxazole if they showed no signs of the disease and their immunity, reflected by their CD4 count, was normal.
The new findings suggest all HIV-positive children should be given the antibiotic, even if their CD4 count is normal.
All of the children in the trial are now on preventative co-trimoxazole and those who needed antiretroviral therapy are now starting it through the Zambian government scheme.
Dr Gibb said: "Tackling HIV infection directly is just one approach to management.
"Reducing the secondary complications and infections, which can be just as fatal as HIV itself to those with weak immune systems, is also important and, as this trial proves, can have dramatic results.
"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."
She said the British government's continuing commitment to this kind of research was of vital importance.
International Development Secretary Hilary Benn said: "The UK government is committed to reversing the spread of HIV and Aids internationally.
"Our £1.5bn strategy commits us to taking action to provide prevention, treatment and care, funding medical research and delivering desperately needed support to children orphaned by Aids and vulnerable to HIV.
"This is a breakthrough in medical research which can help to save children's lives all over the world."
A spokeswoman from WHO's HIV department said: "Until you can confirm the child's HIV status they should be taking co-trimoxazole.
"And any HIV infected child should be taking it regardless of their CD4 count.
"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.
There are thought to be 12m Aids orphans in Africa - and a million have the virus.