First it's something happening on the other side of the world. Then it's the country or state next door. Yet weeks still go by, life rumbles along, plans are made.

Then, before you know it, things take off. Within days, a few advisories about hand-washing metastasise into school closures, cancelled sporting events, travel restrictions. Infections, deaths keep piling up – and suddenly you're Italy, or Iran. In total lockdown. Businesses shuttered. Doctors in overstretched hospitals choosing who lives and who dies.