The outbreak started in China, where it quickly engulfed the city of Wuhan before racing across the globe on commercial flights and ships, eventually killing more than 1 million people, over 100,000 of them in the U.S.

The novel virus triggered a state of emergency in New York City; caused so many deaths in Berlin that corpses were stored in subway tunnels; overwhelmed London’s hospitals; and in some areas of France left half of the workforce bedridden. Severely ill patients suffering from acute pneumonia were put on ventilators, often in vain. It was the late 1960s, and the Hong Kong flu was sweeping the world.

That pandemic raged over three years, yet is largely forgotten today, a testament both to our resilience and to how societies are now approaching a similar crisis in a much different way.

German Chancellor Angela Merkel, British Premier Boris Johnson and President Emmanuel Macron of France have described the coronavirus pandemic as their countries’ greatest challenge since World War II. Mr. Macron described it as a war.

But scientists and doctors say the Hong Kong flu is a more apt comparison. And because it happened in recent times—unlike the more devastating and better remembered Spanish Flu of 1918—it can offer lessons for today, though experts disagree on what these might be.