The five largest commercial health insurers in the U.S. have contracted merger fever, or maybe typhoid. UnitedHealth is chasing Cigna and even Aetna; Humana has put itself on the block; and Anthem is trying to pair off with Cigna, which is thinking about buying Humana. If the logic of ObamaCare prevails, this exercise will conclude with all five fusing into one monster conglomerate.