Scientists suggest people with more copies of a certain gene are less likely to become infected with the HIV virus or to develop Aids.
Researchers looked for copies of a particular gene
The US National Institutes of Health team say their finding could lead to a screening test to determine someone's susceptibility to HIV/Aids.
The gene the researchers studied is one which helps the body to fight the HIV virus.
The research was published online by the journal Science.
The gene encodes CCL3L1, a protein which blocks HIV which also interacts with CCR5 - a receptor protein that HIV uses as a doorway to enter and infect immune system blood cells.
The scientists analysed blood samples from more than 4,300 HIV-positive and -negative African-Americans, Europeans and Hispanic-Americans to determine the average number of copies of the CCL3L1 gene in each group.
It was found that there was significant variation. HIV-negative African-American adults had an average of four CCL3L1 copies, while HIV-negative European- and Hispanic-American adults had around two and three copies respectively.
The researchers say this does not mean European Americans are more prone to HIV/Aids than the other groups studied.
Instead, they say they found individuals with fewer CCL3L1 copies than their population's average were more susceptible to infection and rapid progression to Aids, and therefore people with a larger than average number of CCL3L1 copies were less at risk.
It was found that each additional CCL3L1 copy could reduce the chances of acquiring HIV by up to 10.5%.
Below-average numbers of CCL3L1 copies were associated with an increased risk of rapid Aids onset of between 39% and 260%.
This is not the first research to suggest people's genes could determine their susceptibility to HIV/Aids.
One study even suggested that people whose mother and father were both descended from ancestors who survived the bubonic plague in the seventeenth century appeared to be immune to HIV.
Dr Anthony Fauci, director of National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, which funded the research, said: "Individual risk of acquiring HIV and experiencing rapid disease progression is not uniform within populations.
"This important study identifies genetic factors of particular groups that either mitigate or enhance one's susceptibility to infection and disease onset."
He added: "These differences in the immune system might result in medical approaches to thwart HIV/Aids or other infections that vary among groups."
Michael Carter, patient information editor at the UK's National Aids Manual, said: "The field of genetics is likely to be a fruitful area of research into how we can prevent and treat HIV/Aids.
"It will be particularly important in treatment, where we will be looking at tailoring treatments dependent on people's genetic make-up."
He said doctors could already assess whether a patient was likely to experience side-effects from a particular treatment based on their genetic profile.
Yusef Azad, director of policy and campaigns at the National Aids Trust, said: "The research published today could well be an important step forward.
"We need the global community to provide the political commitment and funding necessary to ensure that all such avenues are fully pursued."