A 26-year-old male with a habit of drinking untreated water presents with persistent high fever (40°C) for 3 weeks, accompanied by abdominal distension. Physical exam: T 39.3°C, P 104 bpm, lethargy, soft/flat abdomen, liver and spleen palpable at the costal margin. Laboratory tests: WBC 3.4×10<sup>9</sup>/L (neutrophils 51%, lymphocytes 49%). Widal test shows 'O' agglutination 1:160 (+) and 'H' agglutination 1:320 (+). The most likely diagnosis is: