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A 2-month-old male infant was admitted with a 2-day history of cough and fever. Respiratory pathogen testing on admission confirmed influenza A (H1N1) virus infection. The patient was started on oral oseltamivir for antiviral therapy along with supportive symptomatic care. After 3 days, his fever resolved and coughing improved, but he developed diarrhea. The stools were yellowish-green, watery and loose, occurring 4–5 times per day without mucus or blood. Routine stool examination, stool bacterial culture and Clostridium difficile testing showed no abnormalities. Following routine treatment, diarrhea worsened progressively, with frequency increasing to 14–16 times daily. Stool volume was large, totalling 280–320 g per day, and peaked at 480 g on one occasion.