I repeatedly point this out. There was the church choir practice where 45 out of 60 people were infected within what two hours? There is also a Korean call center where about 2/3 of the people in an open office were infected.

I've repeatedly linked to CDC study that estimates r0 in Wuhan before lockdown at between 3.8 and 8.7. Median 5.8.

Take away: Completely avoid being indoors with large groups of people.