I suppose it depends all manner of things - how long the flight is, is the infected person/carrier walking around/sat in a row of 3/coughing etc etc but I'd imagine a 9 hour flight from Europe to the U.S would almost certainly mean someone with the virus will infect others.
Agree! I suspect that they make these decisions on probability. In the example of this 2nd patient in NC, they flew from Italy to JFK and then JFK to Raleigh. They specifically talked about the small regional jets that go from NYC airports to RDU. That's when she talked about the specific number of seats/rows that they deem at risk. My takeaway is that they aren't going to say "this patient was on flight DL123" and freak out everyone from that flight and even if you were on his flight, unless you were in those X number of seats, they probably aren't going to be calling you.