Humanity is now in the midst of its fastest-ever period of change. There were almost 2 billion people alive in 1918; there are now 7.6 billion, and they have migrated rapidly into cities, which since 2008 have been home to more than half of all human beings. In these dense throngs, pathogens can more easily spread and more quickly evolve resistance to drugs. Not coincidentally, the total number of outbreaks per decade has more than tripled since the 1980s.

Globalization compounds the risk: Airplanes now carry almost 10 times as many passengers around the world as they did four decades ago. In the ’80s, HIV showed how potent new diseases can be, by launching a slow-moving pandemic that has since claimed about 35 million lives. In 2003, another newly discovered virus, sars, spread decidedly more quickly. A Chinese seafood seller hospitalized in Guangzhou passed it to dozens of doctors and nurses, one of whom traveled to Hong Kong for a wedding. In a single night, he infected at least 16 others, who then carried the virus to Canada, Singapore, and Vietnam. Within six months, sars had reached 29 countries and infected more than 8,000 people. This is a new epoch of disease, when geographic barriers disappear and threats that once would have been local go global.

Last year, with the centennial of the 1918 flu looming, I started looking into whether America is prepared for the next pandemic. I fully expected that the answer would be no. What I found, after talking with dozens of experts, was more complicated—reassuring in some ways, but even more worrying than I’d imagined in others.