Get yourself tested. Pretend you've never heard of the girl.
Assuming you are negative, DON'T tell your wife. Sometimes, telling the 'truth' to get something off your chest is more about making YOURSELF feel better than doing what's right for the other person.
But, you need to promise yourself to NEVER do it again & keep that promise. Living with any guilt IS your penance, if you need to think of it that way. If you can't do this tho, you should leave your wife & let her be free to find someone better.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh