Some states even have laws about it, in general you can be held criminally and or civilly liable for pretty much any deliberate attempt to hurt someone or harm caused by gross negligence, STDs are no exception, in general though to people having sex consensually without protection are both viewed as being negligent. Prisoner's with HIV have been charged and convicted of Aggravated Battery for attempting to splash their blood on corrections officers, that case is from Illnois which actually has a Felony called "Criminal Transmission of HIV', but in general the law doesn't differentiate much between guns, knives, and bio-weapons.