When I had mono, the docter told me that I could have been carrying it for a while and only got sick because I was under a lot of stress at the time.
The virus that causes infectious mononucleosis is Epstein-Barr, and by the time you get to your 30s, nearly everyone (in the United States, at least) has been exposed to it at least once and is therefore a carrier.
In young children, active illness resembles any other minor viral illness; it's "self-limiting," meaning it goes away on its own like the common cold.
If a person avoids exposure to EBV as a child and only becomes exposed during adolescence or young adulthood, roughly half will still be asymptomatic and/or very mildly ill, but the other half will get the swollen glands and paralyzing fatigue that everyone thinks of when they hear mono. It's not wildly common to get to adolescence without being exposed to EBV, so most people go through life without ever getting really sick from it.