If someone receives oral sex, can they contract an STD? So can the person giving oral sex have a STD orally and spread it to someone’s genitals? Which STDs can be transmitted this way and how likely is this? I received oral sex very briefly (maybe a few seconds) and now am concerned I could have contracted an STD.  I do not have any signs or symptoms.  I also have had oral HSV 1 for over 20 years. Does that make it impossible to contract HSV 1 genitally since I already have it orally?

Oral sex is a lot less risky for STDs than vaginal or anal sex. And cunnings (oral-vaginal contact) is lowest risk of all; it is very rare for a woman to acquire any STD by oral contact with her vagina. In theory, it could happen with gonorrhea and syphilis, but probably never for chlamydia and extremely rarely for HPV. (If you perform oral sex on a male partner, you could transmit an oral STD to him; or could acquire an oral STD yourself, especially gonorrhea. However, even these are quite uncommon compared with vaginal or anal sex.

As for HSV1, you definitely are not at risk, for the reason you almost state yourself. People with either HSV type are immune (or at least highly resistant) to new infections with the same type, anywhere on the body. Your oral HSV1 makes it extremely unlikely you could catch genital HSV1, even if you have an oral sex partner who has oral herpes. (For the same reason you cannot transmit your own oral infection to your genital area.)

All things conquered, I would not advise you be tested for anything on account of this single exposure.

I hope these comments are helpful. Let me know if anything isn't clear. 

HHH, MD