Can you get HIV through oral sex?
Receiving Oral Sex
If you are receiving oral sex from someone else you are only being exposed to saliva. The concentrations of the virus in saliva are so low that nobody has ever been infected from saliva. Keep in mind, however, that you can get other sexually transmitted diseases (like herpes) by receiving oral sex. However, as far as HIV is concerned, receiving oral sex is extremely low risk.
Giving Oral Sex
If you are giving someone oral sex, there is a risk of infection, since pre-cum, semen, vaginal secretions and menstrual blood can get into your mouth. The more of these body fluids you are exposed to, the greater the risk of infection there would be. If you have any open sores, cuts, abrasions or gum disease in your mouth, the virus can get into your bloodstream. The risk is less than if you had vaginal or anal intercourse, but the risk is real, and transmission can occur. There have already been reported cases of HIV infection specifically through giving oral sex. In addition to HIV, while giving oral sex, you could also be at risk for other sexually transmitted diseases (STDs) including herpes and gonorrhea and even syphilis.
Now, when we're talking about the levels of risk when you give someone oral sex, several variables actually determine the true level of risk. Let's look now, for example, at the risks of giving a man oral sex:
Both pre-cum and semen can contain high concentrations of HIV. Semen, however, is a riskier body fluid, because you are normally exposed to a greater quantity of semen as compared to pre-cum. Does that mean that pre-cum is totally safe? No! But we can say that the more infectious body fluid you are exposed to, the greater the likelihood of transmission. So, you can become infected by pre-cum alone, but you are much more likely to become infected if the guy cums in your mouth, since you're then exposed to a much greater quantity of his body fluid.
Of course, the virus must also be able to get into your bloodstream through some type of open sore, abrasion, gum disease, etc. The more openings that HIV has to get into your bloodstream, the greater your risk would be. So the more cuts or open sores in your mouth, the greater the risk would be. Or if a person has gum disease, the more severe the gum disease is, the greater the risk would be.
Without ejaculation, there still is some risk of getting infected through giving oral sex, but the risk would be much greater if the man ejaculated in your mouth. So rather than saying high risk vs. low risk, it's actually a spectrum of risk.
No exposure to pre-cum or semen: No risk as far as HIV is concerned.
Exposure to pre-cum only: Low risk (but still technically possible). The more pre-cum you get exposed to, the greater the risk would be.
Exposure to both pre-cum and semen: Risky, especially if there are cuts/open sores in the mouth. The more semen you're exposed to, and the more cuts/abrasions/gum disease in the mouth, the greater the risk. But overall, although risky, it is still generally considered less of a risk than unprotected intercourse.
So again, we're talking about a spectrum of risk. This is why there will be no absolute answer of high vs. low risk of giving oral sex. But we can say that HIV has now been found to be transmitted by giving oral sex especially if there is ejaculation but not receiving oral sex.
And by the way, a very important thing to remember is that there doesn't necessarily have to be ejaculation to be infected with other STDs. For example, if you give a man oral sex, and that man has gonorrhea, you could get infected with gonorrhea in your throat, whether the man ejaculates or not. Gonorrhea can cause a discharge that can be very infectious if it gets into the throat (or penis/rectum/vagina) of another person. So things that may be lower risk for HIV (giving oral sex without ejaculation) may be high-risk for other diseases, like gonorrhea.
Regarding the risks of giving a woman oral sex:
Keep in mind that if you are giving a woman oral sex, the exact same principles apply as described above, that is, the more vaginal secretions or menstrual blood that you get in your mouth, and the more cuts and open sores you have in your mouth, the greater the chance of infection.