I think that would be my reaction as well.
I'd like to think I'd be more calm about it but I really don't know.
IMO, knowingly infecting someone is the worst thing you can do to someone. Murder is morally superior in my eyes. At least with murder you are ending their life right there. By infecting knowingly infecting someone, you are basically damning that person to live out the rest of their existence as a diseased person no one would ever sleep with as they die slowly from the virus you gave them.
It is FUCKED UP. One of my friends got an STD from his bitch of an ex. She knew she had it and didn't tell him. I don't know what I would have done in that situation.
I think the first thing to do is not to assume that the person "knew they had it" and gave it to you on purpose. That's a pretty rare circumstance and I'd have to see real evidence or a confession before I'd believe it. And if it's not intentional and they didn't get it as a result of cheating on you, then there's really no one to blame. Like I said, we could all insist on mutual and broad STD testing before having sex with a new partner. How many of us do?