HIV infection through oral sex alone seems to be very rare.
Oral sex poses very low, theoretical (not likely to happen, only might happen) risk for transmission.
Saliva is known to contain virus, but, in very low concentrations, not enough to cause infection.
When transmission occurs through this route, additional factors have contributed, such as having oral ulcers, bleeding gums, genital sores and other STD's.
Protected vaginal sex in couples, where one partner is HIV-positive and the other partner, negative, poses less than 1% HIV transmission
Correct and consistent use of latex or polyurethane condom is highly effective way to protect from HIV (virus doesn't pass through condom barrier)