An 8-year-old male presents with persistent high fever for 12 days, accompanied by diarrhea (3-5 times/day) and vague right lower abdominal pain. Physical exam: liver palpable 2 cm below the costal margin, spleen palpable 1.5 cm below the costal margin. Laboratory tests: WBC 2.5×10<sup>9</sup>/L (band neutrophils 10%, segmented neutrophils 60%, lymphocytes 30%), stool shows few WBCs and pus cells, ALT 100 U/L, total serum bilirubin 48 μmol/L, anti-HBs positive, Widal test (O 1:160, H 1:160), Leptospira agglutination test 1:200, urinary protein (+), urobilinogen (+), urinary bilirubin (-). The most likely diagnosis is: