Yes, STIs can be transmitted during unprotected oral sex. Not using a condom, dental dam, or other barrier puts all partners at risk. This means that performing or receiving oral sex puts you at risk. Some STIs are more likely to be transmitted during oral sex than others, including:
Herpes
Herpes is transmitted by skin-to-skin contact with a developing or existing sore. The virus can be transmitted from mouth to genitals if the person giving oral sex has or is developing a cold sore. It can also be transmitted from genitals to mouth if the person receiving oral sex has or is developing a genital sore.
Gonorrhea
Gonorrhea is transmitted when bacteria are present in body fluids. A person giving oral sex can get a gonorrhea infection of the throat if their partner has gonorrhea.
HPV
HPV is transmitted through skin to skin contact and can be transmitted from the vagina, penis or anus to the mouth or vice versa. The risk for HPV-related oropharyngeal cancers (cancers of the throat, tonsils and base of the tongue) is believed to be linked to the number of oral sex partners that someone has had.
Syphilis
Syphilis can be transmitted through performing or receiving oral sex. The painless sores and other symptoms of this infection can be subtle, particularly in the mouth, so it is fairly common to be unaware of an infection.
HIV/AIDS
For the person receiving oral sex, there's little chance of contracting HIV, although it's difficult to pinpoint when HIV has been transmitted because people rarely engage in only one type of sexual activity. In the case of fellatio (oral sex on a man), the HIV virus theoretically could gain entry from the mouth to the opening on the tip of the penis, or through an open cut or lesion on the penis. If you receive oral sex, however, you mainly expose yourself to saliva, which has negligible concentrations of HIV.
For the cunnilingus (oral sex on a vulva) recipient, the chance of HIV transmission is also low, although the entire vagina is a mucous membrane through which, theoretically, the virus can be transmitted. A person receiving cunnilingus is more at risk of getting herpes or gonorrhea from their partner than HIV.
The risk of HIV infection is greater for the partner who performs oral sex. A person performing oral sex on a vulva should avoid it when their partner is menstruating, since menstrual blood can carry the HIV virus. Research presented at the 7th Conference on Retroviruses and Opportunistic Infections in February of 2000 concluded that 8 of 122 cases in an HIV-transmission study were possibly attributable to performing oral sex on a penis. Of these 8 infected people, some reported having had recent dental work or having cuts in their mouths, suggesting that HIV transmission by oral sex is associated with cuts, lesions, or irritation of the tissues in the mouth.
Other STIs that can be transmitted through oral sex, although less commonly, include:
