Mononucleosis, or "mono," is a common illness caused by a virus. It's often seen in teens and young adults. It can cause a bad sore throat, swollen neck glands, and fever. Some people feel tired and weak for weeks or months after getting sick.

Mono usually goes away on its own, but rest and good self-care can help you feel better. Mono is nicknamed "the kissing disease" because that's one way the virus is spread. If you have mono, don't kiss anyone or share things like drinking glasses, eating utensils, or toothbrushes.