After chiding China's over secrecy, Trump White House attacks coronavirus whistleblower - 2020-02-28
So a Health and Human Services (the CDC's parent agency) official raised a concern about that agency's handling of the return of Americans returning from Wuhan China, specifically that those officials — all non-medical professionals — were ordered to to receive the evacuees without protective gear, ordered to enter quarantined areas where the evacuees were present, and then weren't ever tested for the virus. One of those staff members stayed at a nearby hotel and left California on a commercial flight.
So it's probably not a coincidence that the first American case of "unknown origin" appeared a week later near Travis Air Force Base, where the evacuees arrived.
As a reward for bringing this to the attention of the Office of the Special Counsel, the anonymous official was retaliated against and reassigned, and told she'd be fired if she didn't accept the reassignment.