According to WebMD, the Epstein-Barr virus which most often causes mononucleosis, is spread through contact with saliva or mucus. It is often referred to as the kissing disease because it is easily spread through kissing.
WebMD states that mononucleosis is a common illness that causes a high fever, a sore throat, swollen lymph nodes or glands, weakness and general fatigue. Symptoms typically begin four to six weeks after initial exposure to the virus and can last for many weeks or months. Mononucleosis can also cause a person's spleen to swell and burst in rare cases. A ruptured spleen is a medical emergency and should be treated immediately.