The Department of Justice is digging in its heels against UnitedHealth Group, alleging in a second lawsuit the health insurance giant has knowingly scammed the government by inflating the medical codes of its Medicare Advantage members.

But UnitedHealth is not the only Medicare Advantage insurer under the federal microscope — it's just the largest, covering about one-fourth of all people in the program. The DOJ also is investigating Humana, Centene and other insurers for their coding practices.

The big picture: Medicare Advantage is the private version of Medicare that has more limited networks of doctors and hospitals but cheaper premiums, and it is heavily consolidated. The four biggest companies — Aetna, Humana, Kaiser Permanente and UnitedHealth Group — control 56% of the market. Any scrutiny or changes in payment policies will affect those insurers most.