A 30-year-old male presents with intermittent chills, high fever, and sweating for over 2 weeks, visiting in October. Asymptomatic during intervals. Physical exam: temperature 40.5°C, mild anemic appearance, herpes labialis, heart rate 120 bpm (regular rhythm), liver 1 cm below costal margin, spleen 2 cm below costal margin (soft and tender). Laboratory tests: Hb 89 g/L. The most likely diagnosis is: