Could you forgive?

I thought that my SO had never lied to me. We tell each other everything, but then I found out I have gonorrhea. He swears that he never cheated but I'm not sure how else this could have happened. We have been together for 3 years and I was tested back in May (negative everything). We have a 5 month old together. This is super hard for me because other than this, he has never given me a reason to not believe him. But he has to be lying, right?