Covering the American health care system means we tell some scary stories. This episode of “An Arm and a Leg” sounds like a real horror movie. It uses one of Hollywood’s favorite tropes: machines taking over. And the machines belong to the private health insurance company UnitedHealth Group. Host Dan Weissmann talks to Stat News reporter Bob Herman about his investigation into Medicare Advantage plans that use an algorithm to make decisions about patient care. The algorithm is owned by a subsidiary of UnitedHealth Group. Herman tells Weissmann that some of UnitedHealth’s own employees say the …