Here's what can happen in Illinois if you claim not to have herpes, but your partner ends up with it...
A woman sued a guy she met through "It's Just Lunch." They went on two dates in 2011 and both "affirmed that neither had any sexually transmitted diseases." They had sex and she began experiencing symptoms of Herpes II virus. (HSV-2)  She sued him for emotional distress, fraud, misrepresentation and other things. He responded that he had been tested in 2010 and was negative for HSV-2 and attached the 2010 lab results.  The trial Court ruled that she could force him to undergo a "visual and manual physical exam, including a discussion of his medical and sexual history" followed by a blood test for the virus, because he had put his medical condition at issue when he submitted his 2010 results.  There were other issues raised which will likely be the subject of other appeals.  This ruling of the Illinois Appellate Court  was issued 2/24/15, and can be viewed at  2014 IL App. (1st.) 133424.
Of course, the opinion doesn't say if they practiced protected sex or what either of them did between his 2010 testing and their 2011 sexual experience...