HIV can stay in needles for weeks
Children in South Africa are being infected with HIV through dirty needles, experts have claimed.
Researchers have suggested hundreds of thousands of children may have contracted the virus in this way.
The study is the latest to point to contaminated needles as a major cause of HIV in Africa.
Some researchers believe as many as 40% of HIV infections in African adults are linked to injections.
United Nations agencies have rejected this theory, saying most cases are linked to unsafe sex.
Officials have also warned that the theory could damage campaigns to get people in Africa to use condoms to protect themselves from the disease.
This latest research looked at a study carried out by the Human Sciences Research Council of South Africa, published last year.
It revealed that 5.6% of South African children between the ages of two and 14 have HIV. This represents 670,000 children.
The matter needs to be much more closely interrogated before we form conclusions
Dr Nono Simelela,
South African HIV and Aids programme
However, figures for mother-to-baby transmission - believed to be the main cause of HIV in children - are substantially lower.
This suggests children are contracting the virus in another way.
Researchers from the University of Tübingen in Germany said the findings indicated contaminated needles were to blame.
They rejected claims that children could have contracted HIV through unsafe sex or as a result of abuse.
Writing in the Journal of Obstetrics and Gynaecology, they said: "For hundreds of thousands of South African children to have acquired HIV sexually, inordinately high levels of childhood sexual exposure would be required, a phenomenon unlikely to have been overlooked by paediatricians. Recent reports from South Africa discourage this hypotheses."
The researchers also examined other studies. They said these also showed differences between HIV rates in children and the number of mother-to-child transmissions.
They said the findings showed unsafe sex was no longer the main cause of HIV in Africa.
"The common belief that HIV transmission in Africa is driven by heterosexual exposure is no longer tenable," they wrote.
"There is mounting evidence that rapid HIV transmission is fuelled by parenteral exposures in health care settings, especially medical injections but also including transfusion of untested blood and others.
"Not only are injections popular among African patients, administered at an estimated 90% of medical visits, but also often unnecessary and injection equipment is often used."
The researchers said urgent action is needed to improve standards in South African clinics.
"They must educate their patients in the dangers of non-sterile injections and ensure that their own practices are beyond reproach."
They said the findings could also be applied to other countries on the continent.
"We must protect patients from their own medical care system in all countries with similar epidemiological characteristics."
South Africa has the largest HIV population in the world - one in five people are infected.
However, the South African government rejected claims children were contracting HIV through dirty needles.
Dr Nono Simelela, head of its national HIV and Aids programme, told BBC News Online: "I have worked in clinics and hospitals in various part of our country - including some that were really poorly equipped.
"But nowhere have I seen practices that would make me conclude that dirty needles are the most probable explanation for this surprising rate of HIV-infection in children. And I am confident my doubts would be shared by many other clinicians.
"I believe the matter needs to be much more closely interrogated before we form conclusions about the cause."