Epstein–Barr virus
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.