Mononucleosis, or "mono", is an infection caused by the Epstein-Barr virus. The virus spreads through saliva. Mono occurs most often in 15 to 17-year-olds, but you can get it at any age. Symptoms of mono include fever, sore throat, and swollen lymph glands. Most people get better in two to four weeks, but may feel tired for a few months afterward.