BBC News Online examines why Aids has spread so fast in Africa amid claims that it has more to do with unsafe medical care than unsafe sex.
What is the current situation in Africa?
The most recent figures from UNAids show a continent divided in two. In north Africa, up to 0.5% of the population has been infected with HIV.
In sub-Saharan Africa, that figure is 8.8% but rising to almost 40% in some countries. In four countries - Botswana, Lesotho, Swaziland and Zimbabwe - at least one in three of the population is HIV positive.
According to UNAids, an estimated 30m people in sub-Saharan Africa have HIV - 58% of these are women.
There were approximately 3.5m new infections last year, representing 70% of the global total. That figure is continuing to rise.
The situation is now so bad in some countries that it is contributing to the spread of famine as key workers are lost to the disease.
Why has HIV taken such a hold in Africa?
Most experts believe that sexual contact is responsible for 90% of HIV transmissions in Africa.
There has been a long history of unsafe sex coupled with promiscuity in many countries. There has also been a well-documented lack of awareness about the disease across the continent, which experts say fuels the spread of HIV through unsafe sex.
What is the latest theory?
A study by researchers in the United States suggests two out of three new infections are caused by contaminated needles.
They have reviewed hundreds of studies on HIV transmission across Africa dating back 20 years.
They believe most cases are linked to poor hygiene in hospitals, with patients being injected with needles contaminated with the virus.
Is this theory plausible?
The theory is plausible but very controversial. The researchers say their review found evidence that people who have never been exposed to HIV through sex had contracted the disease.
These included women who appeared to be HIV-negative before giving birth and children whose mothers were also free from the disease.
It is a fact that HIV can be passed on through contaminated needles. HIV can survive in syringes at room temperature for more than four weeks.
The risk of contracting HIV following injection with a contaminated needle has been estimated at between 2% and 7%. The average person in the developing world receives 1.5 injections each year.
Do aid agencies buy this argument?
Some organisations have reacted angrily to this latest research. They have warned that it could dilute the safe sex message which they are trying to spread across Africa.
UNAids insists that unsafe sex is the main route of transmission. Its scientific advisors warn this latest theory could cause some people to drop their guard against HIV and to stop using condoms.
UNAids estimates that contaminated needles are responsible for 5% of HIV infections and not the 60% put forward by this latest research.
However, it acknowledges that hygiene standards in many hospitals in some parts of the world are not up to scratch.
Its officials say they would need $290m to ensure a clean needle for every medical treatment or vaccination in the world in two year's time.
This controversy over how HIV is spread in Africa comes amid a major international row over ensuring people across the continent have access to cheap anti-retroviral drugs.
UK Chancellor Gordon Brown stepped up the pressure on pharmaceutical companies this week calling on them to make sure cheap drugs are made available to those countries in most need.