A 26-year-old woman had a 4-day history of fever, aches, and throat irritation. She had tender right posterior cervical lymphadenopathy and mild hepatosplenomegaly. Her hemoglobin was 12.6 g/dL, white blood count 5.5 × 109/L with relative lymphocytosis (58%), and platelet count 100 × 109/L. The peripheral smear showed reactive lymphocytes with rare (1%) apoptotic lymphocytes (top left panel shows apoptosis with nuclear condensation and bleb; top right, with nuclear condensation only; bottom left, with bleb formation; and bottom right, with nuclear fragmentation). Liver function tests were moderately abnormal. Malarial parasite, dengue, blood culture, viral hepatitis markers, and Widal were negative. The frequency of apoptotic lymphocytes increased to approximately 10% by the fifth day. Infectious mononucleosis was considered, but a monospot test was negative. The patient was treated symptomatically. The fever persisted, and a repeat peripheral smear still showed reactive lymphocytes wi