For a time, it looked like our defenses might hold.

The extraordinary lockdown of China’s Hubei province over the past month had at times appeared to be halting the spread of the Covid-19 coronavirus. For much of that period, new cases outside China were running at just a dozen or so a day, most of them with clear connections back to the epicenter in the city of Wuhan.

Perhaps this latest infection could follow the path of severe acute respiratory syndrome? The SARS strain of coronavirus vanished within months…