I think that the larger digital companies like Amazon, Facebook, Google and the like were early in shutting down their offices in favor of social distancing. The governor's order for the three largest counties with almost half the state's population also was key.
See
Newsweek article here,
ER Visits for Coronavirus-Like Illnesses Drop in Washington Hospitals
The portion of my quote they did not use was, "If you develop difficulty in breathing, go to the hospital immediately." I've heard that people can be doing OK, even getting better, and then crash.
Eric Holdeman is a contributing writer for Emergency Management magazine and is the former director of the King County, Wash., Office of Emergency Management.