Mononucleosis, often called “mono,” is a viral infection that can cause flu-like symptoms. The infection is sometimes called the “kissing disease” because it is highly contagious and lives within an infected person’s spit. One of the earliest symptoms of mono is an intense feeling of fatigue, or tiredness. Mono can affect anyone, but is most commonly diagnosed in adolescents and people in their early 20s. So, how exactly do you know if you have ...