Mono!!! It's the ORIGINAL kissing disease; otherwise known as
infectious mononucleosis, I managed to pass it on to my SO after a few weeks of dating. Evidently, I had mono sometime during my childhood and didn't realize it; people who have had mono continue to shed active Epstein-Barr virus (the cause of infectious mononucleosis) periodically throughout their lifetimes.
Hopefully you've had mono already.