Federal health authorities said Tuesday they now expect a wider spread of the coronavirus in the U.S. and are preparing for a potential pandemic, though they remain unsure about how severe the health threat could be.

Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention, said Tuesday the agency expects a sustained transmission of the virus and called for businesses, schools and communities to brace themselves and plan for potential outbreaks....