In February, a massive cyberattack nearly brought down the entire U.S. health system. Doctors are still reeling, and many patients don’t even know their data has been exposed. Today, Dan Diamond traces what went wrong and the new scrutiny in Congress.Read more:Even if UnitedHealthcare isn’t your health insurer, the company has probably interacted with you or your data in some way. UnitedHealth Group is both the nation’s largest insurer and its largest employer of physicians. It owns pharmacies and home health agencies. One of its subsidiaries, Change Healthcare, processes more than 40 percent of the country’s medical claims, acting as a kind of “information superhighway,” explains the Post’s national health reporter, Dan Diamond.  In February, hackers broke into that system and led to what is being described as the largest cyberattack ever in American health care. Behind the scenes, the attack froze health payments and compromised patient information. It spread pain across doctors and hospitals nationwide, especially in rural communities. It’s still unclear how many people have been impacted, and the breach has yet to be fully resolved.   The chaos and fallout brought UnitedHealth Group’s CEO, Andrew Witty, to testify this week before Congress for the first time in more than 15 years. During separate House and Senate committee hearings, representatives grilled Witty on why basic security safeguards were lacking and, more broadly, whether UnitedHealth Group might have become too big, raising bigger questions about how U.S. health care operates. Today’s show was produced by Elana Gordon. It was edited by Lucy Perkins and mixed by Sean Carter. Thanks also to Stephen Smith.  Subscribe to The Washington Post here.