The spread of the Roman Empire through Europe could help explain why those living in its former colonies are more vulnerable to HIV.
The claim, by French researchers, is that people once ruled by Rome are less likely to have a gene variant which protects against HIV.
This includes England, France, Greece and Spain, New Scientist reports.
Others argue the difference is linked to a far larger event, such as the spread of bubonic plague or smallpox.
The idea that something carried by the occupying Romans could have a widespread influence on the genes of modern Europeans comes from researchers at the University of Provence.
They say that the frequency of the variant corresponds closely with the shifting boundaries of the thousand-year empire.
In countries inside the borders of the empire for longer periods, such as Spain, Italy and Greece, the frequency of the CCR5-delta32 gene, which offers some protection against HIV, is between 0% and 6%.
Countries at the fringe of the empire, such as Germany, and modern England, the rate is between 8% and 11.8%, while in countries never conquered by Rome, the rate is greater than this.