After decades of progress, life expectancy — long regarded as a singular benchmark of a nation’s success — peaked in 2014 at 78.9 years, then drifted downward even before the coronavirus pandemic. Among wealthy countries, the United States, in recent decades, went from the middle of the pack to being an outlier. And it continues to fall further and further behind. Chronic diseases are the greatest threat, killing far more people between 35 and 64 annually, but where and why?