The Epstein-Barr virus is a common virus related to the herpes family of viruses. Like herpes, most people are exposed to it at some point in their lifetime. However, the body naturally produces antibodies to the virus and in most cases people infected with it suffer mild symptoms (similar to a cold) or no symptoms at all and never develop symptoms again after the first infection.
However, in developed nations, most individuals are not continuously exposed to the virus. As it is generally transmitted from direct mouth to mouth contact, children are rarely exposed so it often is encountered for the first time by adolescents and young adults in developing countries and in about half of cases exposure will lead to infectious mononucleosis, a contagious, inconvenient but rarely fatal disease that will run its course, but only after a period of several weeks.