What Caused COVID-19? What We Do (and Don't) Know 5 Years Later

Five years ago today the World Health Organization called COVID-19 a pandemic for the first time. Much about the coronavirus was a mystery then – how exactly it spread, if we could treat it and whether we could prevent it. While the answers to those questions unraveled over the following years, a big one remains unanswered: What is the origin of COVID-19?Theories about the origin of the coronavirus that has killed more than 1.2 million Americans have sparked strong debate – much of it seemingly partisan – in the U.S. and beyond. Leading theories include an animal host that transmitted...