Genetically-engineered bacteria could be a cheap and effective method of protecting women from HIV infection.
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Researchers from Stanford University in the US found a bacterium which occurs naturally in the vagina.
By subtly altering its genetic structure, they made it bind onto HIV and prevent the virus from infecting human cells.
The researchers say that, in time, treating women in developing countries with the bugs could slow HIV spread.
All women have bacteria which exist naturally in the vaginal tract - and do no harm to their "hosts".
For scientists struggling to find a way to stem the increase in HIV in the developing world, they could offer women some way of controlling their own protection even if condoms were not used.
The interest centres on a bacterium called Lactobacillus jensenii, which was genetically engineered so that it produced a protein dubbed "2D CD4".
This protein was designed to latch onto any passing HIV and hold onto it.
The theory is that any HIV particle taken out of circulation this way is one less HIV particle which can go on to infect the cells lining the vagina.
In practice, while the bacteria has not been tested in the human population, in the laboratory, the engineered Lactobacillus managed to inhibit the entry of HIV into similar cells in a test tube.
The researchers, writing in the journal Proceedings of the National Academy of Sciences, described the findings as an "important first step" towards finding a bacteria HIV-blocker.
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The advantage of the bacterial method, they said, was that unlike condoms or chemicals, the bugs could persist for weeks or months at a time, offering protection throughout this period.
UK experts are also impressed. A spokesman for Aidsmap said that the idea appeared promising.
"The technology explored in the paper does seem to have potential - not least because it looks like it will be easy to apply which will be durable and should be completely unobtrusive during sex and daily life - key to the success of any microbicide.
"We'll have to see how it pans out in properly designed human trials - which will take a very long time so it's going to be many years before any product emerges from this research.
"A modest reduction in HIV infectivity is reported - that initially seems to be a concern, but it should be borne in mind that a modest reduction in infectivity may be all that is needed to prevent many cases of HIV transmission."