Anthem is buying rival Cigna for $48 billion in a deal that would create the nation’s largest health insurer by enrollment, covering about 53 million U.S patients. In just three weeks, starting with Aetna’s $35 billion bid for Humana Inc. on July 3, the landscape of U.S. health care has been altered in a buyout frenzy that could transform five massive U.S. health companies into just three, including UnitedHealth Group.