It was a late afternoon on a blue-sky day in Jiujiang, a city that borders Hubei, the central Chinese province worst hit by the coronavirus.

We were riding in our taxi along a main road en route to our next destination, a large bridge over the Yangtze River, which we heard had been shut down by a checkpoint as transport links with Hubei were all but severed.

Then we noticed a police car driving alongside us. A voice over the car’s loudspeaker ordered our driver to pull over...