So A While Back I Had Sex With A Friend

So a while back I had sex with a friend and wound up with chlamydia (we used a condom but here I am). Anyway, due to the unlikelihood of having anything, I, over the period of six months slept with two other men (unprotected). When the first informed me he had chlamydia, I was tested and came back positive so I informed the other two who got tested and came back negative. Now the second could very well have been treated and denied it but the last man and I had sex only a day before he was tested. I'm just wondering how it's possible for me to have an STD, have unprotected sex and not pass it on to my partner?