Mayo Clinic Health Manager
Get free personalized health guidance for you and your family.
Get StartedCauses
By Mayo Clinic staffThe cause of mononucleosis is the Epstein-Barr virus, although similar signs and symptoms are sometimes caused by cytomegalovirus.
Mononucleosis usually isn't very serious. Most adults have been exposed to the Epstein-Barr virus and have built up antibodies. They're immune and won't get mononucleosis again.