A natural ingredient of human blood has been shown to block HIV, raising hopes of a new class of drugs to treat infection with the virus.
HIV replicates within cells
The molecule works in a way different to current antiretroviral therapies, and so could be a new line of attack.
Researchers found its potency could be boosted by making tiny changes to its chemical structure. It also worked against drug resistant HIV strains.
The University of Ulm study appears in the journal Cell.
According to the latest World Health Organization estimates, nearly 40 million people worldwide are living with HIV/Aids.
Close to 4 million people became infected with HIV in 2006, and the virus was responsible for about 3 million deaths last year alone.
Antiretroviral drugs have played a key role in helping to keep the majority of people with HIV who take them alive.
But HIV has an astonishing ability to modify its structure, and there are concerns that many of the major drugs currently in use will become increasingly ineffective.
For that reason it is important that scientists come up with new alternatives.
Many molecules in the blood have been thought to have some sort of inhibitory effect on HIV, but it has not been clear which are significant.
The researchers used state-of-the-art technology to assess the anti-HIV impact of more than one million blood proteins.
They found fragments of the key molecule, which they call virus-inhibitory peptide (VIRIP), are relatively abundant.
Tweaks to its amino acid components boosted its anti-HIV potency by two orders of magnitude.
Tests also showed that some derivatives of the molecule are highly stable in human blood plasma, and non-toxic even at very high concentrations.
A synthetic version of VIRIP also proved effective at blocking HIV, excluding the possibility that some other factor was responsible.
VIRIP targets a sugar molecule which HIV uses to infect a host cell.
Roger Pebody, a treatment advisor at the Terrence Higgins Trust, said: "This is early stage research but may be very useful in developing a new class of HIV drugs.
"Many people with HIV become resistant to current treatments over time so it is vital we keep working on expanding people's options.
"It may take years, but let's hope that this leads to an effective future treatment for HIV."