The US team reviewed hundreds of studies on HIV transmission cases
The United Nations has disputed the findings of United States researchers which says most HIV infections in Africa result from dirty medical needles.
The suggestion that the spread of the virus that can cause Aids is closely linked to unsafe medical care challenges widely held scientific views.
The research estimates that about 60% of people with HIV in Africa become infected mainly through contaminated needles rather than through sexual contact, but the UNAids organisation puts the figure at nearer 5%.
UNAids says the conclusions they have drawn are not supported by adequate studies.
Catherine Hankins, UNAids chief scientific adviser, expressed her concerns.
"Unsafe sex continues to be the major route of transmission throughout the world," she said.
We're concerned that a report like this might tend to make people drop their guard and not use condoms
UNAids chief scientific officer Catherine Hankins
"We're concerned that a report like this might tend to make people drop their guard and not use condoms, when it's exactly using condoms that is required at this point.
The organisation does say that more resources are needed to ensure sterile medical care in all countries, not just the industrialised ones.
It would cost $290 million to ensure a clean needle for every medical treatment or vaccination in the world in two years' time, research shows.
Dr Christopher Uoma, HIV co-ordinator for ActionAid in Kenya, said he had not a chance to study the full research but was initially shocked by the findings.
"It could have profound implications for our programme and Africa in general," he said.
"It could lead to a serious change in terms of health behaviour with people being reluctant to enter hospitals."
He also warned that it could encourage some people to revert to previous habits of risky sexual behaviour.
He pointed out that HIV epidemics in South Africa and Zimbabwe, which had good health systems, were less developed than those in countries such as the Democratic Republic of Congo, where medical care was poorer.
This was the opposite of what would be expected if most cases were transmitted through medical procedures, he said.
The US researchers reviewed hundreds of studies on HIV transmission across Africa, going back 20 years, and concluded the main cause was the use of dirty needles for medical injections.
The research, published in the International Journal of Sexually Transmitted Diseases and Aids, was funded privately by members of the team.