I started masking on my public transit commute in Jan 2020 by pure coincidence at the time - I remember news of "stuff" coming out of China in December. My friend returned from visiting family in Wuhan in late January and said there were some things that were odd there (relating to flu/illness), and she started masking on our commute, I frequently think about how the first confirmed US death (at the time) was in Santa Clara – literally where we commuted from (Caltrain to SF)