The novel coronavirus now epidemic in China has features that may make it very difficult to control. If public-health authorities don’t interrupt the spread soon, the virus could infect many thousands more around the globe, disrupt air travel, overwhelm health-care systems, and, worst of all, claim more lives. The good news: There’s still an opening to prevent a grim outcome.

China failed to contain the virus early. More cases in the U.S. are inevitable. Experience with the 2009 H1N1 flu pandemic suggests that emergency measures...