Researchers found that people who live in lands conquered by the Roman army have less protection against HIV than those in countries they never reached

They say a gene which helps make people less susceptible to HIV occurs in greater frequency in areas of Europe that the Roman Empire did not stretch to.

The gene lacks certain DNA elements, which means HIV cannot bind to it as easily and is less able to infect cells.

People with the mutation have some resistance to HIV infection and also take longer to develop AIDS, reports New Scientist.

A study of almost 19,000 DNA samples from across Europe showed the gene variant seemed to dwindle in regions conquered by the Romans.

Generally only people in Europe and western Asia carry the gene and it becomes much less frequent as you move south.

More than 15 per cent of people in some areas of northern Europe carry it compared with fewer than four per cent of Greeks.

It is not clear why this is so since the spread of HIV - which began in the early 1980s - is too recent to have influenced the distribution of the variant.

The difference in frequency of the key gene mutation reflects the changing boundary of the Roman Empire between 500 BC and AD 500.

But study leader Dr Eric Faure, of Provence University in France, does not believe the Romans spread the regular version of the gene into their colonies by breeding with indigenous people.

Dr Faure, whose findings are published in Infection, Genetics and Evolution, said: "Gene flow between the two was extremely low."

Instead he believes the Romans introduced a disease to which people carrying the gene variant were particularly susceptible. As the Romans moved north this disease killed off people with the variant gene that now protects against HIV.