Mononucleosis, known as the "kissing disease," is a viral infection that causes fatigue, fever, sore throat and swollen lymph glands. Mono is usually contracted when a person gets exposed to an infected individual's saliva, usually via kissing or sharing eating utensils. Treatment centers around the relief of symptoms, but otherwise it will take approximately 10 days for the fever to subside and 4 weeks for any other symptoms to go away. Fatigue can linger for several months afterwards.