In 2015, I asked Bill Gates a simple question: What are you most afraid of?

He replied by telling me about the death chart of the 20th century. There’s the spike for World War I. The spike for World War II. But between them sat a spike as big as World War II. That, he said, was the 1918 Spanish flu pandemic, which killed an estimated 65 million people. Gates’s greatest fear was a flu like that, ripping through our hyperglobalized world.