“Yes.”

She looked at me pointedly, then asked him, “Brandon, how big is the hippo?”

Brandon held up his thumb and forefinger about an inch apart. “This big.”

Brandon’s tiny hippo wasn’t just the product of an imaginative kid. It was a blaring announcement that perhaps we were treating something that didn’t exist. For over a year I had watched him carefully, jumped at every cough, and trained him to report any new symptoms.

And he was ill. He just didn’t have asthma.

My son was one of the more than 12 million Americans who experience a diagnostic error each year, a number equivalent to the combined populations of New York and Los Angeles. According to a report published by the Institute of Medicine in September, almost everyone in the U.S. “will experience at least one diagnostic error in their lifetime, sometimes with devastating consequence.” Those numbers are based on a 2014 study that found that around one out of every 20 outpatient experiences results in a misdiagnosis—people receive the wrong information about the cause of their illness, or they have treatment delayed because of an error, or they’re treated for something they don’t have.

Experts agree that these estimates are likely conservative. Other studies have shown error rates from 10 to 48 percent, depending on the specialty. Because the diagnostic process can involve many players and moving parts, identifying the true rate of error is nearly impossible. “There are no health-care organizations tracking diagnostic errors that I know of, anywhere,” says Mark Graber, the president and founder of the Society to Improve Diagnosis in Medicine. “The tools that hospitals have aren’t set up to detect diagnostic errors.”

And the small amount of research that exists is almost entirely limited to adult patients—meaning that as little as we know about the full extent of misdiagnoses, we know even less about how they affect children’s health.

We do know this much: Misdiagnoses in kids happen fairly often—perhaps even more than in adults (according to the IOM report, children, like patients with mental illness, can be especially challenging for diagnosticians). While pediatricians aren’t sued for malpractice as often as most other specialists, they do get sued for misdiagnosis far more often than any other group. When The Doctors Company, a large medical-liability insurer, compiled its data from 2007 to 2013 for the IOM, it found that misdiagnoses accounted for an astonishing 61 percent of all malpractice suits against pediatricians. And in a 2010 survey of more than 1,300 pediatricians in the journal Pediatrics, 54 percent admitted to making a diagnostic error at least once or twice each month.

One potential factor may be the sheer breadth of knowledge that pediatricians need in order to practice: Besides assessing sick children, they have to know about the physical, neurological, and emotional development of children at every stage, from newborns to teens. They’re also gatekeepers to almost every other specialty.