Medicare plans to downgrade ratings on some of the Humana's health insurance plans. The stock has fallen 23% for its worst weekly performance since 2009.

Humana Inc. investors haven’t struggled through anything this bad since the global financial crisis 15 years ago.Most Read from BloombergThe Corner Store ComebackNYC Schools Reverse Course on Cell-Phone Ban After Parents BalkWhat Do US Vehicle Regulators Have Against Tiny Cars?

Shares of Humana have tumbled after the health insurer said a Medicare Advantage quality rating drop will hurt future bonus payments the company receives.