When it comes to mononucleosis, causes of the illness include two viruses: the Epstein-Barr virus (EBV) and the cytomegalovirus (CMV). In 85 percent of all mononucleosis cases, EBV is the cause of the illness. CMV is the cause of mononucleosis in the other 15 percent of cases. Both of these mononucleosis causes are part of the herpes family of viruses.
Two viruses can cause mononucleosis (mono): the Epstein-Barr virus (EBV) and cytomegalovirus (CMV). Both viruses are part of the herpes family, whose other members include the viruses responsible for cold sores and chickenpox.
EBV causes mononucleosis in 85 percent of cases. When EBV is the cause of mononucleosis, it is also referred to as infectious mononucleosis. About half of all children are infected with EBV before they're 5, but at that young age, it usually doesn't cause any symptoms of mono. If you don't become infected with EBV until you're a teen or older, you're more likely to develop mono symptoms.
After you're infected, the virus stays with you for life, but usually doesn't cause any additional symptoms. Still, every now and then you may produce viral particles in your saliva that can transmit the virus to other people, even though you feel perfectly fine. By age forty, 85 to 90 percent of Americans have EBV antibodies, indicating that they have the virus in their system and are immune to further EBV infection.
CMV is also a very common virus. About 85 percent of the US population is infected with it by the time they reach adulthood. As with EBV, CMV frequently produces no symptoms, and mono most often results when infection occurs in the teens and 20s. Sore throat and swollen lymph glands are less common in people who have CMV mono than in those infected with EBV. When CMV causes mononucleosis, it is also referred to as mononucleosis syndrome, or CMV mononucleosis.