A. Although mono is usually thought of as the 'kissing disease' that only teens get, you can actually get mononucleosis at almost any age. In fact, outside the United States, over 90 percent of children have mono by the time they are five years old.
In the United States, children usually become infected with mono as teens or young adults, when they have classic symptoms, including high fever, sore throat, swollen lymph glands, fatigue, and headache, which may last a week or two.
Younger children usually have much more mild symptoms though. So while a teen might have a high fever and bad sore throat for a week or more, an infant might have no symptoms or mild symptoms, that according to the CDC might be 'indistinguishable from the other mild, brief illnesses of childhood.' So they may just be a little irritable, with a decreased appetite, or they may have mild upper respiratory infection symptoms, like a cough, runny nose, and mild fever.
In fact, the symptoms are so mild that no one even suspects that these children might even have mono.
So how do you know that they had mono then?
Often, it is not until sometime much later when a child is sick with a fever and sore throat and is thought to have mono and testing is finally done. This testing, including antibody testing for the Epstein Barr virus, may then reveal the past infection.