Much speculation about the risks of oral sex has concentrated on the likelihood of infection through cuts in the mouth, but it is equally plausible to argue that the key factor in determining infection through oral sex is viral load in semen or vaginal fluids. The quantity of infectious fluid which enters the mouth may also play a role. If this is the case, it would suggest that fellatio is more risky than cunnilingus.

American epidemiologist James Koopman has calculated that the probability of HIV transmission through oral sex during the early weeks of infection is considerably greater than at later stages in HIV infection. He based his calculations on the assumption of much higher viral load in semen during this period (see The mechanisms of HIV transmission for discussion of the evidence). Koopman also argues that virtually all cases of HIV infection can be attributed to transmission during primary infection.

Koopman's statistical analysis does not allow for an increase in infectivity later in infection, and is also subject to potential bias because of the small size of the sample of seroconverters he used to calculate the probabilities of infection by different routes.

Nevertheless, the continuing low level of HIV infection through oral sex may be attributable entirely to infections acquired during primary infection or during symptomatic HIV disease. If this is the case, it raises a number of issues:

• HIV–positive people who have low CD4 counts and potentially high viral load should be aware that they can infect their partners if they ejaculate in their mouth during oral sex

As greater numbers of people become symptomatic, the likelihood rises that infections will occur by this route

• The changing risk and reality needs to be acknowledged whilst still highlighting the relatively low risk attached to oral sex. Even if oral sex is risky during primary and symptomatic HIV disease, the remarkably low level of infections attributable to oral sex suggest that the risk remains small.