A 54-year-old white man presented with abdominal pain, vomiting, and diarrhea. His symptoms had started 4 months earlier during a trip to Colorado, when he developed altitude sickness. Since then he had experienced intermittent episodes of nonbloody diarrhea alternating with constipation. A course of metronidazole hydrochloride did not affect the symptoms. On the day of admission, he presented with a severe episode of periumbilical pain and vomiting. His examination was notable for right lower-quadrant tenderness with a distended abdomen and a white blood cell count of 9320/μL.