Infectious mononucleosis -- known popularly as "mono" or "the kissing disease" -- has been recognized for more than a century. An estimated 90 percent of mononucleosis cases are caused by the Epstein-Barr virus (EBV), a member of the herpes virus group. Most of the remaining cases are caused by certain other herpes viruses, particularly cytomegalo virus. This fact sheet focuses on mononucleosis caused by EBV. EBV is a common virus that scientists estimate has infected over 90 percent of people aged 40 or older sometime during their lives. These infections can occur with no symptoms of disease. Like all herpes viruses, EBV remains in the body for life after infection,