My three year old had mono and we never even new she had it until we had to get blood work done because her tonsils were swollen. It turned out that she did not have mono at that time, but she did at some point. Once you have mono, it will always show up on your blood work. We were so surprised that she ever even had it because she did not show any signs of it. The doctor said it is totally different in kids, and they usually do not get all the symptoms that adults get. Very weird in my opinion!
Updated
My three year old had mono and we never even new she had it until we had to get blood work done because her tonsils were swollen. It turned out that she did not have mono at that time, but she did at some point. Once you have mono, it will always show up on your blood work. We were so surprised that she ever even had it because she did not show any signs of it. The doctor said it is totally different in kids, and they usually do not get all the symptoms that adults get. Very weird in my opinion!