I'm no doctor, but I had mono about six years ago. I don't know how I got it. I think I was staying up too late and not getting enough sleep which caused me to get sick. I saw a specialist who told me that mono USED TO BE CALLED the kissing disease. He acted like it was a myth. I had a boyfriend at the time and he never got it. He was stubborn and wanted to kiss me while I had mono anyway, which he did, and luckily he never got sick. Personally, I don't think it's a good idea. You can catch a cold from kissing someone who has a cold, so it's definitely possible. Just because your girlfriend has mono does not mean she kissed someone else.