The “Spanish” flu of 1918 was one of the deadliest pandemics in human history. Seeming to come from nowhere in the waning days of World War I, it spread through a war-ravaged world like wildfire. In a matter of months, a third of the world’s population was infected, 50 to 100 million people died, and the global economy shrank by 5%.

The world of 1918 was very different from today’s, where the new coronavirus is emerging as another potential pandemic. A century ago, antibiotics, modern hospitals, intensive care units and instant...