Add Layer to an existing map. In the top left click on Import and simply drag KML data into the browser window.
The New York Times is making Covid-19 data publicly available on GitHub. Fairfax Co provides daily updates but not historical data that I could find. I downloaded historical Covid data for all US counties provided by NYT, entered it into Numbers, selected out the Fairfax data and graphed the daily numbers. I plan to tweet the daily numbers since the county is no longer sending out txt alerts.