Jennifer Schreiber took one look at her patient’s gray and blue skin and knew the teenager needed to go to the hospital.

On July 11, the 18-year-old arrived at Dr. Schreiber’s office, struggling through quick, shallow breaths. Within 10 minutes, emergency medical services were called, and the girl was wheeled to the intensive-care unit at Children’s Hospital of Wisconsin in Milwaukee. She was fighting for her life.

The...