Anthem has 40 million customers
The records of tens of millions of people may have been taken in what is likely the largest breach involving a US health insurer.
Hackers have breached the IT system of health insurer Anthem and stolen personal information relating to current and former consumers and employees.
The company, America's second largest health insurer, has 40 million customers.
The cyberattack did not appear to involve medical information or financial details such as credit card or bank account numbers, Anthem said.
However, information accessed during the "very sophisticated attack" did include names, birthdays, social security numbers, street addresses, email addresses and employment information, including income data, the company said.
Anthem said it immediately made every effort to close the security vulnerability and reported the attack to the FBI.
The company did not say how many customers and staff were affected, but the Wall Street Journal reported it was suspected that details of tens of millions of people had been taken, which would likely make it the largest data breach involving a US health insurer.
Security experts say cyber criminals are increasingly targeting the $3tn US health care industry, which has many companies still reliant on ageing computer systems that do not use the latest security features.
Medical identity theft is often not immediately identified by patients or their provider, giving criminals years to milk such credentials.
That makes medical data more valuable than credit cards, which tend to be quickly cancelled by banks once fraud is detected.
Twitter Boss: We 'Suck' At Dealing With Trolls
Dick Costolo promises to start "kicking off" trolls "right and left" in a memo to staff at the social network.
Twitter chief executive Dick Costolo sent a memo to staff
Twitter's chief executive says the company "sucks" at dealing with trolls and he is "ashamed" of how poorly it has dealt with the issue.
Dick Costolo said in recent memos to employees that bullying behaviour could be driving users away, and he has promised tougher action to deal with abusers.
He wrote: "We suck at dealing with abuse and trolls on the platform and we've sucked at it for years.
"It's no secret and the rest of the world talks about it every day. We lose core user after core user by not addressing simple trolling issues that they face every day."
The memos follow an internal employee forum in which a staff member asked what could be done to address abuse on the social network.
Mr Costolo promised to start "kicking these people off right and left and making sure that when they issue their ridiculous attacks, nobody hears them".
Twitter recently streamlined the process for reporting abuse, and in November it teamed up with an advocacy group to investigate harassment against women.
According to studies, women are disproportionately affected by online abuse.