With the government shutdown officially taking effect after midnight on Friday— with Democrats and Republicans pointing fingers at each other — most Americans are focusing on the political factors at play. Yet a shutdown could have serious consequences for several of the most important national health agencies, particularly if it drags on for long.

The Department of Health and Human Services is furloughing just over half of its employees. The HHS contingency staffing plan includes 40,959 employees stopping work, while 40,956 continue.

The Centers for Disease Control and Prevention is sending 63 percent of its 13,600-employee staff home. This, during the height of flu season: The CDC reports that every state but Hawaii and the District of Columbia has suffered widespread flu outbreaks, and officials worry about what may happen without this testing.

On Saturday, White House Budget Director Mike Mulvaney told reporters that the CDC will now keep its influenza monitoring program up and running through the shutdown.

Tom Frieden, director of the CDC during the 2013 government shutdown, said recently that he "really couldn't do my job as CDC director of keeping Americans safe, because more than 8,500 of my staffers had been told to go home, and they do important things that protect Americans."

Frieden told STAT News that the agency made him feel like he was "in a science fiction movie. CDC was deserted."