Harry Stevens, at The Washington Post, with my favorite explanation from over the weekend of why this virus is so dangerous and how it spreads:

[...] these simulations vastly oversimplify the complexity of real life. Yet just as simulitis spread through the networks of bouncing balls on your screen, covid-19 is spreading through our human networks — through our countries, our towns, our workplaces, our families. And, like a ball bouncing across the screen, a single person's behavior can cause ripple effects that touch faraway people.

The visuals in this piece are excellent.