Mistakes in US hospitals may be the country's third biggest killer, which is bad news for patients and for trust in the medical system.

• A Johns Hopkins study put deaths from medical error at a quarter million a year, behind only heart disease and cancer.
• Those deaths do not show up in official cause-of-death numbers, so the scale stays hidden.
• Long shifts are a big part of the problem: a doctor 12 hours into a shift is impaired about as much as a drunk driver.
• The same tired doctors are the ones making life-and-death calls, often late at night.

Outlook: Pressure is likely to grow for hospitals to track these deaths honestly and rethink how long doctors work.