Maybe the first documented case. It started in China mid November. China did nothing to stop the spread in their own borders till late Jan. So I'm betting people that were infected but not showing symptoms, or maybe even showing symptoms but didn't know what they had, were already leaving China flying around the world to other countries by early December.
I think there's a few people in this thread that are suspicious they had it back in Jan or Feb. Flu like symptoms and went to doc but didn't test positive for influenza. Rode it out at home and felt like crap for a few days.