MOST PEOPLE HEAD TO THEIR DOCTORS IF THEY HAVE CHEST PAIN or a suspicious lump, but signs like these often appear too late. Catching symptoms earlier requires ongoing monitoring—the kind of thing a cell phone might do. Health-scanning systems that exploit the continuous flow of data from cell phones could help eliminate the perilous lag time between the onset of symptoms and diagnosis. Mobile devices could also help care providers identify and treat problems before they become too serious—and too expensive—to address effectively. In theory, such always-on warning systems could slash the 75 percent of health care spending used for chronic disease management and extend life spans by staving off millions of potential health crises.