Mononucleosis Mononucleosis is an infectious disease of humans in which the blood and tissues contain mononuclear leukocytes (white blood cells with only one nucleus), either monocytes or lymphocytes. An infectious disease is a disease that can give you an infection, can be transmitted by infection without actual contact, or can be caused by a microorganism. All species of animals are afflicted with infections caused by a wide variety of organisms, from submicroscopic viruses to wormlike parasites. When a person has an infectious disease like mono the organism gains access to the patients body, survives, and then multiples. Next, the patient gets the sympto