The thing started in China in December, maybe earlier. US population that travels to China tends to be young, active, connected in business circles. Brushes the thing off like a minor cold, without skipping a beat. So community transmission in the US could have been going as early as December, just not in the circles that are particularly affected by that virus.
Planes evacuating people from Wuhan (also ~47 days ago) were having 1% of virus positives at the time.