Duncan changed planes in DC so I'm expecting a case to pop up in this area soon...my optimistic side hopes that he wasn't spreading it when he was on the plane but the prepper in me says "don't count on it." Then we have the doctor in Bethesda who checked himself in. I'm not too worried about him because he is being smart about it and checked himself in before showing symptoms, he probably doesn't even have it.
I think it's important to differentiate between cases that can be tracked to patient zero and those that can't. Once we start seeing cases that can't logically be tracked to Duncan, we are in big trouble and I believe that will be a true tipping point; this could happen any day now or maybe they will be able to track all cases for a few days, who knows.
I think we are already getting to the point that we will be seeing cases "in the wild" that we have no clue where they came from due to runners, terrorism and incompetence. Duncan rode on several planes too, seems like we should be seeing cases in Europe.
When more than one case starts popping up in DC and/or Baltimore, particularly if they are unable to track it's vector, we will begin self quarantine.