Experience. I suspect the actual # of cases is soooo under-reported that literally hundreds of thousands of Americans have been exposed. Maybe even millions.

Which is why I keep going back to my serology soapbox. At this point I care less about who tests positive (because treatment will largely be based on symptoms, which show regardless of knowing a test result; and because if someone feel ill he's likely gonna self-quarantine for a bit) and more about who has had an An response because it tells us where we really are on a state-by-state and nationwide basis.