A 30-year-old male presents with fever and fatigue for half a month, without chest pain or arthralgia. Examination reveals jugular vein distension, cardiomegaly, distant heart sounds, heart rate of 103/min, and hepatomegaly. Laboratory tests show WBC count 10×10<sup>9</sup>/L, ESR 25mm/h. Echocardiography confirms pericardial effusion. Pericardiocentesis yields 800ml of pale straw-colored fluid with WBC 0.4×10<sup>9</sup>/L (400/mm<sup>3</sup>), neutrophils 40%, lymphocytes 60%. No tubercle bacilli or other bacteria on smear. Which etiology is most likely?