There's no need for a letter. That's NOT something you tell someone in a letter. You have to tell him face to face, because yes, he gave you an STD, and it is BOTH your faults for not using condoms. It is just as important for women to make sure they are protected with condoms.

Chlamydia is a TREATABLE STD that goes away with a pill. No big deal, you could have had something that stays with you forever. Once treated, it goes away. It's a lesson learned to ALWAYS wear condoms.