Exposure to alcohol makes mouth cells more susceptible to HIV infection during oral sex, research has shown.
Earlier studies focused on how alcohol consumption increased the chance of someone having unprotected sex and therefore risking HIV infection.
But the team from the University of California at Los Angeles (UCLA) looked at how alcohol affected cells.
The research is published in the Journal of Acquired Immune Deficiency Syndrome.
Cells from the lining of the mouth, the epithelium, were obtained from people who were HIV negative.
The cells were then exposed to various concentrations of alcohol, similar to those found in beers, and then to a strain of HIV which had been modified with green fluorescent protein so that researchers could see if it infected cells.
It was found that epithelial cells which had been exposed to 4% of ethanol for 10 minutes showed between a three to six-fold greater susceptibility to infection from the HIV strain.
HIV attacks a particular type of white blood cells called CD4+ cells.
White blood cells adhere to endothelial cells.
HIV hijacks the cell, inserting its own genes into the cell's DNA and uses it to manufacture more virus particles. These go on to infect other cells.
The CD4+ host cells eventually die, although scientists do not know exactly how.
The body's ability to fight diseases decreases as the number of CD4+ cells drops, until it reaches a critical point at which the patient is said to have Aids - Acquired Immune Deficiency Syndrome.
Deborah Jack, chief executive of the National Aids Trust, said the UCLA study was useful.
"We always welcome new research into the risks of HIV transmission, but these are very early findings and much more work needs to be done to prove a biological link between alcohol and HIV transmission.
"However, the influence of alcohol on the decision whether or not to have unprotected sex has already been established and particularly at this time of year we urge people to enjoy the festive season safely. "