"It isn't fully understood. People come up with various explanations," one expert said.
One plausible theory is that because only the very tip of the penis is a "susceptible surface", when it comes into contact with an infected woman the risk is small. However, a woman may contract infection across the whole of the cervix. Furthermore, semen may be left in the vagina.
"It's pure biology," the expert said. "There's more surface area there for the woman than for the male.
The risk of male-to-female transmission is thought to be at least twice as high as vice versa, but some studies have suggested that it might be as much as 20 times greater.
However, other factors also come into play. The HIV virus is particularly infectious in the first three or six months after being contracted and in the late stages when the body's immune system is breaking down - although external signs of the disease are not necessarily present.
The risk is higher if there is any other sexually transmitted disease or if there is any "trauma" during sex, for example if blood is drawn. It also increases if the man is circumcised.
Most of the research on the transmission of the HIV virus has been carried out in Europe and the United States into "discordant couples", where only one partner is infected initially but there is continued contact.
The threat from a one-night stand is much more difficult to calculate.
"There are cases of people who appear to have been infected on a single contact," the health expert said. "Certainly it is not a risk that one would recommend to one's friends."
Robin Gorna, of the Terrence Higgins Trust aids charity, said that it would be "extremely unusual" for a woman to infect a man in a one-night stand, but it was not impossible. "Condoms should always be worn," she said.Reuse content