Asymptomatic primary infection with Epstein-Barr virus: observations on young adult cases by Abbott, RJ, Pachnio, A, Pedroza-Pacheco, I, Leese, AM, Begum, J, Long, HM, Croom-Carter, H, Stacey, A, Moss, PAH, Hislop, AD, Borrow, P, Rickinson, AB, Bell, AI