A 16-year-old male presents with muscle strength grade 3 in all four limbs and numbness for 10 days. He had diarrhea 10 days prior, which resolved in 2 days. Examination shows clear consciousness, bilateral peripheral facial palsy, muscle strength grade 2 in limbs, hypotonia, absent tendon reflexes, calf tenderness, and sock-glove pattern sensory loss. CSF analysis: WBC 4×10<sup>6</sup>/L, protein 0.6g/L, glucose 3.2mmol/L, chloride 122mmol/L. If this represents acute motor axonal neuropathy (AMAN) variant of Guillain-Barré syndrome, the most common infectious pathogen is: