Mononucleosis is an infection caused by a herpes virus known as the Epstein-Barr virus, or EBV.

What is going on in the body?

Most people are exposed to EBV during childhood. Like other herpes viruses, it stays quiet in the body long after symptoms go away. It can be reactivated later in life. Often this happens when the immune system has been weakened by disease, cancer treatment, or an organ or bone marrow transplant.

What are the causes and risks of the infection?

Mononucleosis is caused by EVB. The virus may stay quiet in the body long after it is acquired. People with weakened immune systems are at risk for severe disease. This group includes individuals with cancer or immunodeficiency disorders.

EBV is spread primarily through saliva. It is nicknamed the kissing disease. Less commonly, it can be spread via blood transfusion. EBV lives in the person's mouth and throat for years after initial infection. Shedding of EBV by healthy individuals, through contact with salvia, accounts for most of the spread to uninfected people.