You can get an STD through contact with semen in oral sex, but if the person “giving head” has an STD, could the other contract it by means of penile-saliva contact?
Yes, it’s possible to get an STD from receiving oral sex without a condom or dental dam. Some STDs are spread just through skin-to-skin contact (like putting your mouth on a penis or vulva). Others are spread through sexual fluids, like semen or vaginal fluids.
For example, herpes can be easily spread during oral sex, because it’s passed through skin-to-skin contact and not just fluids.
So it's a good idea to get tested for STDs regularly — and be honest with your nurse or doctor about what kinds of sex you have and if you're noticing any symptoms. You can get tested for STDs at your local local Planned Parenthood Health Center. And you can help prevent STDs by having safer sex and using condoms or dental dams during oral sex.