treated for clhymida, partner doesnt have it but another does... how could this be?

Not to long ago i was told that i had clhymida and i got cured from it. but the thing is the person that i last had sex with didnt have it. even though he did admitt to having sex with one other person he said that he used a condom but she had it too... just wondering is it possible for both of us to catch this atd from him and he doesnt have it? ( me and him only had sex once and before him i hadn't had sex in over a year.)
answers plzzz