The message is to practice safe sex and not to worry about being bitten by flies

Professor David Mabey, communicable disease expert

Most blood-sucking insects pose no risk of passing on HIV, including mosquitoes, which inject saliva through one tube and suck up blood through another.

However, stable flies, which bite humans, could be an exception and are known to transmit equine leukaemia virus between horses.

When feeding, they scrape skin to make a wound, suck up blood and regurgitate some on the skin next time they feed.

Any viruses in the regurgitated blood can invade the body through the wounds made by the flies.

Unlike other blood-sucking insects that regurgitate blood, the stable fly does not digest the blood it regurgitates.

Gerhard Brandner of the University of Freiburg, said: "The anterior part of the mid-gut where the regurgitate is kept is just for storage and is free of digestive enzymes.

"That's our key result, and is a precondition for transmission of HIV."

Distractions

If the flies sucked up virus-tainted blood from the chimps, they could transmit it when they feed on humans, they believe.

They speculate that sporadic cases of HIV transmission via stable flies may have happened for years but gone unrecognised.

If these rare infections still happen at all, they now pale into insignificance alongside the explosive spread of HIV through unprotected sex.

It is a point reinforced by Professor David Mabey at the London School of Hygiene and Tropical Medicine.

He said: "It's an interesting theory and no more.

"The trouble with all these stories is that they distract attention from the main public health message, which is 90% of infections are transmitted by sex or from mothers to infants.

"So the message is to practice safe sex and not to worry about being bitten by flies."

Beatrice Hahn at the University of Alabama in the USA, who first reported finding HIV infections in chimps in 1999 does not think the German research should be taken seriously until further research backs it up.

She said: "There's no shortage of hypotheses of how SIVcpz made the jump to humans and this is just another one.

"The task at hand is to find out for sure what happened and back it up by hard evidence."