you tell them with honesty and remorse if you are the one that exposed them. remember that exposure does not always end up causing disease in another person. maybe they won't get it. or maybe they already had herpes and didn't tell you. clearly when you tell someone that you may have inadvertently exposed them to a sexual transmitted disease it will not be easy. both of you are a bit responsible however; since it appears you may not have practice safe sex (no condoms). if you just found out where did you get it? open and honest communication in a private area (not work! ) is important.
i just found out i have herpes. how to tell someone you work with every day you may have given it to them?