The chances of HIV being passed on during fellatio are low, but it can sometimes happen.
On the other hand, several sexually transmitted diseases, including chlamydia, gonorrhea, herpes, syphilis, genital warts (HPV) and hepatitis B can be passed on when either your penis is sucked or you suck someone else's penis.
Even without the exchange of body fluids, many sexually transmitted diseases can be passed on during close or intimate contact. You could use a condom during oral sex to reduce this risk, but you can't completely eliminate the risk
Scientists characterise the risk of HIV transmission through fellatio as far lower than the risk from anal or vaginal sex. But the risk is not as low as zero.
Sucking the penis of a man who has HIV, with ejaculation in the mouth, is an activity that may occasionally involve HIV transmission.
(Other forms of oral sex, including cunnilingus, are extremely low risk for HIV.) When sucking a penis, the risk of HIV transmission rises if there are bleeding gums, oral ulcers, genital sores or other sexually transmitted diseases.
It's important to have regular check-ups for sexually transmitted diseases, especially if you've had several sexual partners since you were last tested. If you're feeling anxious about a recent sexual encounter that may have exposed you to HIV, the best way to deal with the issue is by taking an HIV test.