Dr. Sarah Kim stood in the hospital toxicology lab, staring at the test results on her computer screen with growing alarm; she had ordered a comprehensive screening on Lee Chung after his unexplained cardiac collapse, expecting to find perhaps elevated potassium or some metabolic issue, but what the tests revealed was far more sinister.

She picked up the phone and called the hospital's chief of staff. "We have a situation with patient Lee Chung; I'm seeing levels of thallium in his blood that ...