Few firms in America are more unloved than health insurers. As gatekeepers of the world’s costliest health-care system, their miserly response to claims is a constant source of patient unhappiness. Investors, by contrast, have long regarded them as soothingly safe bets: boring businesses with steady returns. That is no longer the case. UnitedHealth Group, the country’s largest insurer, stunned investors in April by reporting unexpectedly disappointing results. Within weeks it had replaced its boss and scrapped its profit forecast for the year. Its latest results, released on July 29th, offered more misery. Since November UnitedHealth’s market value has collapsed from $575bn to $240bn.