A 26-year-old male medical missionary to Asia developed the sudden onset of severe, frequent, watery diarrhea. The stool was a gray liquid without odor, blood or pus. He developed extreme thirst and fainted on the way to the infirmary. On exam, he was afebrile, with a blood pressure of 80/60, and he was profoundly dehydrated. He improved with IV fluids but continued to have about 1 L / h of watery stool.

Which of the following is the most likely source of his infection?