We don’t yet know the full ramifications of the novel coronavirus. But three crucial facts have become clear in the first months of this extraordinary global event. And what they add up to is not an invocation to stay calm, as so many politicians around the globe are incessantly suggesting; it is, on the contrary, the case for changing our behavior in radical ways—right now.

The first fact is that, at least in the initial stages, documented cases of COVID-19 seem to increase in exponential fashion. On the 23rd of…