There are six of these biggest health insurance companies: WellPoint, CIGNA, Aetna, Humana, United Healthcare and BlueCross BlueShield, although the latter works on a state-by-state basis. The five biggest health insurance companies insure approximately half of the insured population or well over 100 million people. Here we take a look at the five biggest health insurance companies. WellPoint, Inc. WellPoint has 34 million members in its affiliated health plans and more than 70 million insurees when its subsidiaries are included. This makes WellPoint the largest health insurance company in terms of membership.