Among FEMA's many authorities…

The Restoration Act (formerly Insurrection Act),10U.S.C.§§331-335 (2007), authorizes the President to use the Armed Forces to suppress an insurrection, domestic violence, unlawful combination, or conspiracy in any State; enforce the laws of the United States; suppress rebellions against the authorities of the United States; and restore public order.

FEMA authority:

Executive Order 13295, 68 Fed. Reg. 17255 (2003), Revised List of Quarantinable Communicable Diseases, specifies certain communicable diseases for regulations providing for the apprehension, detention, or conditional release of individuals to prevent the introduction, transmission, or spread of suspected communicable diseases.