STD's and STI's in a committed relationship

My SO and I have been together for a year and a half. We're both very young as far as all this goes (16) and he's been having serious health concerns lately. All these problems have come to no definite conclusion and the search is leading to the consideration of STD's. He believes that he may have contracted an STD but we are only sexually active within the relationship and never had any partners beforehand. I guess my ultimate question is... Can one be born with an inherited STD and never show symptoms?