People living in countries once ruled by the Roman Empire may be more susceptible to HIV infection, NewScientist.com reports.

According to the article, those hailing from countries such as England, Greece, France and Spain are less likely to carry a gene variant—called CCR5-Delta32—that produces a protein receptor HIV cannot bind to. Those who carry this variation have shown to have some resistance to infection, with slower progression to full-blown AIDS than those without it.

Researchers are unsure as to how this genetic difference originated, but some speculate that occupying Romans may have introduced a contagion that people with the variant were particularly susceptible to. Gene prevalence suggests that as the Romans moved their empire north, the disease may have wiped out people with the variant. More than 15 percent of people in certain areas of northern Europe carry the genetic variation compared with 4 percent of Greeks