Well, that's it, isn't it? People refusing to isolate, reduce social contact, maintain social distance, wear a mask, etc. In Hong Kong, with a population density of 6,800 people/km^2 - practically stacked like cordwood - and a population 7,500,000, they've managed to keep case numbers down to 7,900.
Just to be clear, I'm not picking on the US. While Canada may be doing better, we don't have anything to brag about. Well, the Atlantic provinces do...