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He suffered from nausea, diarrhea, abdominal pain, bloody stool, and fever for almost three weeks before doctors finally figured out what was wrong.

The answer may seem almost funny—but by the time the mystery was solved, it had caused an infection that nearly killed him.

What was the problem? The 18-year-old, a professional athlete who was on the road for training with his team at the time, had swallowed a 3-inch toothpick from a sandwich he ate.

His case was recently reported in the New England Journal of Medicine. As the New York Times reports, toothpicks are often swallowed without incident, but a 2014 analysis of 136 serious cases found almost 10% were fatal. The wood picks aren’t affected by stomach acid or digestive enzymes, and they’re hard to see on medical scans.

In the athlete’s case, he visited doctors and hospitals multiple times over the course of his illness, but everyone was stumped; blood tests, a CT scan, and an MRI offered no clues.

Finally, as his symptoms became even more severe and his temperature jumped to 105 degrees, doctors performed a colonoscopy and found the toothpick. It had poked through the man’s intestinal wall and pierced an artery, letting bacteria into his bloodstream and causing severe bleeding.

Blood started spurting from the artery once the toothpick was removed, and the patient was rushed into surgery to repair his intestine and artery; ultimately, a 1.2-inch segment of the artery had to be removed due to damage and replaced with a vein from the patient’s thigh.

(An inhaled toy caused trouble 40 years later.)

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