Mononucleosis is an infection of Epstein Barr virus. Incubation period is 5 to 7 weeks. Symptoms include headache, generalized malaise and fatigue, sore throat, fever, and enlarged cervical lymph nodes. CBC done during the first week of illness shows granulocytopenia with normal white cell count. Thereafter, the white blood cell count increases steadily reaching as high as 50,000 mainly due to increases in lymphocyte count. Heterophil antibodies and Monospot tests are positive. The lymphocytes in IM have large vacuoles, foamy cytoplasm and dark chromatin in the nucleus, thus named "atypical lymphocytes".