The conquering legions of ancient Rome may have helped to spread HIV and Aids throughout modern Europe, research suggests.

Scientists believe the invading Romans caused the loss of a genetic shield that makes some people resistant to infection by the Aids virus, HIV.

The gene variant, called CCR5-Delta32, impairs the ability of HIV to enter white blood cells.

People with the mutation are not as easily infected by the virus and take longer to develop full-blown Aids.

Generally, only Europeans and western Asians carry the variant, which becomes less widespread as you move south.

More than 15% of people in some parts of northern Europe have CCR5-Delta32, compared with fewer than 4% of Greeks.

Why this should be has puzzled scientists. But now researchers have found that the changing frequency of the variant reflects the changing boundary of the Roman empire from 500 BC to 500 AD.

A French study of nearly 19,000 DNA samples from across Europe showed that the number of people with the gene variant dwindled in regions conquered by the Romans.

Research leader Dr Eric Faure, from the University of Provence in Marseille, has come up with an intriguing theory to explain the link.

He does not think the Romans spread a different version of the gene into their colonies by breeding with the natives.

A more likely explanation, he believes, is that they introduced a fatal disease to which carriers of the CCE5-Delta32 variant were unusually susceptible.

New Scientist magazine reported yesterday: "As the Romans moved north, this disease killed people with the variant.

"Faure notes that the Romans introduced cats and donkeys to Europe which may have carried pathogens that spread to humans.

"What's more, the Romans inadvertently brought with them disease-carrying mosquitoes."

It is possible the disease in question was West Nile virus, or something like it.

Modern people with the CCR5-Delta32 mutation are more susceptible to the mosquito-borne infection, the New Scientist reported.