Epstein–Barr virus

The Epstein–Barr virus (EBV), also called human herpesvirus 4 (HHV-4), is one of eight viruses in the herpes family. It is one of the most common viruses in humans.
EBV is best known as the cause of infectious mononucleosis (glandular fever). It is also associated with some forms of cancer, such as Hodgkin's lymphoma, and conditions associated with human immunodeficiency virus (HIV). EBV may be associated with a higher risk of certain autoimmune diseases. Some 200,000 cancer cases per year may be caused by (or associated with) EBV. 
Infection with EBV occurs by the transfer by mouth (oral transfer) of saliva and genital secretions. 
Most people become infected with EBV and gain adaptive immunity. In the United States, about half of all five-year-old children and about 90 percent of adults have evidence of previous infection. Infants become susceptible to EBV as soon as maternal antibody protection disappears. Many children become infected with EBV, and these infections usually cause no symptoms or are just mild, brief illnesses of childhood. In the United States and other developed countries, many people are not infected with EBV in their childhood years. When infection with EBV occurs during adolescence, it causes glandular fever 35 to 50 percent of the time.
EBV infects B cells of the immune system and epithelial cells. Once EBV's initial infection is brought under control, non-active EBV stays in the person's B cells for the rest of their life.