Mono is a very common viral illness in young children; they usually don't get it as severe as teens or adults. In fact, most people have had mono as a child and that is why they don't get it as a teen or adult (luckily, since it's much worse when you get it older). Almost 100% of adults who are tested for mono immunity will show that they had it in the past, even if they don't ever remember having had it (they had it as a child and it went unnoticed, like a cold). Like any virus, you can only get it once although there is some research that links "chronic fatigue syndrome" to fluctuating levels of active mono virus in someone who had it in the past but that is controversial.