Got a Problem? Tell me about it and ill give you a solution.
Did you just cheat and because of it got a STD? If you did then dont tell you partner. when she starts to realize something is wrong and she goes to her doctor for a check-up go with her. When they get the result saying that she has it, then you can lay the blame on her by telling her that you will have a check-up yourself. When you get the results go to her and lay the blame on her for giving you the disease.
(Please note that you must be already training to run a marathon in case she finds out.)