Infectious mononucleosis is caused
by the Epstein-Barr virus (EBV). A milder form is caused by the cytomegalovirus (CMV).
Both EBV and CMV are members of the herpes virus family.
In the U.S., most adults have been infected by age 30 with the EBV.
This is a very common virus. When children are infected with it, they often don't
any noticeable symptoms. But uninfected teens and young adults who come in contact
the virus may develop infectious mononucleosis.
Even after the symptoms of infectious mononucleosis have gone away,
the EBV will stay dormant in the throat and blood cells throughout that person's life.
The virus can reactivate from time to time in the saliva or blood. But it almost always
does not cause symptoms.