A federal judge Tuesday blocked the merger of two health insurance giants, Aetna and Humana, upholding the Justice Department's verdict that the proposed $37 billion deal would hurt competition. U.S. District Court Judge John Bates wrote in his opinion that the proposed merger would have decreased competition substantially in the Medicare Advantage market in 364 counties. Aetna spokesman T.J. Crawford Bates based his decision enjoining the merger on evidence of "overwhelming market concentration figures" the merger would generate, plus head-to-head competition between Aetna and Humana that would be eliminated if the deal were finalized.