Yes, it is illegal: if someone knows that he/she has a sexually transmitted disease and, despite knowing of it, fails to notify or advise a romantic partner before being physically intimate, then--1) if the partner contracts the disease, he or she could sue the person who knowingly infected him or her for, potentially, a considerable sum of money; he or she would retain a personal injury attorney to help with the suit (there is no lawsuit without infection, because in that case, there is no injury to sue for or be compensated for); and2) it is possible that the partner could also look to press criminal charges for assault--to do this, the partner would contact the police and/or prosecutor's office.