Me and my boyfriend have been together since November of 2020. In April of 2021 he got a call from his doctor after a routine physical that he has Chlamydia. I got tested 3 days after the day he found out & a week later the test results came back negative for me. I still took antibiotics regardless but I know I have not been with anyone else and he promises that he has not either. He has ejaculated in me since December of 2020 & he still swears to this day that he had to have gotten right before me. I just wanted to know what is the likelihood of this happening even though we had been having unprotected  intercourse for almost 5 months? 

This is a fairly common situation, i.e. only one parter in a monogamous couple testing positive for chlamydia. There are a few likely explanations. Of course a very common one is that the infected partner recently had sex with someone else and the infection was not yet transmitted to his or her main partner -- in other words, that your partner is not being truthful with you. You're the only one who can judge this -- I don't know him or your relationship. But there are other explanations entirely unrelated to sexual infidelity. If you have a strong relationship and you have no reason to distrust him, you should believe him now.

Women carry chlamydia longer than men do, often several months, sometimes a year or more. So it is possible you were infected before your relationship began about a year ago and your partner acquired his infection from you. At the same time, your immune system could have cured your infection before you were tested. Or your test was false negative -- i.e. you were infected but the test missed it. (Chlamydia tests in women miss up to 10% of infections.) Men don't tend to carry chlamydia as long as women do, but in theory it's possible your partner was infected before you became a couple, and you were just lucky and not yet infected. (Chlamydia and other STIs are not transmitted 100% of the time.)

At this point, you'll never know for sure which of you was infected first, when, or from whom. The main thing is that you both needed treatment for chlamydia, and I'm very glad to hear you were treated despite your negative test result. If neither of you is still having symptoms (like urethral or vaginal discharge), this is all a done deal and all is well.

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

HHH, MD