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Five years ago, COVID-19 touched down on U.S. soil when a man from Washington state flew home after a trip to Wuhan, China. But for days, officials didn’t know he was carrying the virus because the U.S. at the time wasn’t testing airplanes. Today, the U.S. Centers for Disease Control and Prevention is trying to catch viral intruders before they spread. Dr. Celine Gounder explains.