First, I wanted to say thanks for providing this forum! I'm late 20s/M/USA and recently became exclusive with a F partner who is now in a different city for the summer. I had an unprotected ONS with a girl in her late 20s this past weekend (she said she'd been tested for STDs "recently" and was clean). I'm due to see my partner in 3 weeks and then again around 2 months from now when we return to school. Given the 3 week buffer, I'm not as concerned about giving her chlamydia/gonorrhea and am more focused on STIs with a longer testing window. So my questions are: 

1) With this information: when would you continue unprotected sex with your partner? Is 3 weeks too early / not worth the potential risk? 

2) If I get tested clean at ~2months, what is the chance that something develops after then? 

3) I've seen some of your responses to the chance of contracting HIV (rate in population * rate of transmission). Is there something similar for syphilis or herpes? Those two worry me the most!

I start with an uncertainty:  I'm unaware of the abbreviation ONS, don't find anything helpful when I google it, and don't really know the nature of the events with your weekend partner. However, on the assumption that it includes some sort of unprotected vaginal, anal or oral penetration, I would still say you had an exposure unlikely to result in any STD. Some general principles:  even the most sexually active singles (especially in the US or other industrialized countries) usually have no transmissible STD; your partner's recent negative test results suggest she does not; and even when STDs are present, generally they are inefficiently transmitted -- that is, most exposures don't result in transmission. In addition, you presumably have had no symptoms over the next 3 weeks, which further reduces the likelihood you were infected.

As for "longer window" infections, the only two for which testing is advised are syphilis and HIV. Both are almost zero likelihood in this situation and I don't advise testing for them. Absence of a penile sore (chancre) further reduces the chance of syphilis. But if you'd like the reassurance of negative test results, you could have blood tests for both about 6 weeks after the event.

To your specific questions:

1) While of course there are no guarantees, if somehow I were in a situation like yours, I would continue unprotected sex with my wife without worry of infecting her, and not be tested for anything. OK, maybe gonorrhea and chlamydia (urine test), but even this is low priority.

2) If negative for gon/chl, HIV and syphilis at 2 months, there would be essentially zero risk you have anything at all.

3) There are fewer data on which to provide such estimates for syphilis compared with HIV, primarily because syphilis is entirely absent in the large majority of population groups. But really this should not be a worry at all. The combination of her low risk, your lack of symptoms, and a negative blood test at 2 months will be 100% proof you don't have it.

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

HHH, MD