But Dr. Nancy Messonnier, the director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention, got a day ahead of the plan. At noon on Feb. 25, just as Mr. Trump was boarding Air Force One in New Delhi for his flight home, she told reporters on a conference call that life in the United States was about to change.

“The disruption to everyday life might be severe,” she said. Schools might have to close, conferences could be canceled, businesses might make employees work from home. She had told her own children, she said, to prepare for “significant disruption to our lives.”

The stock market plummeted, cable news blared apocalyptic headlines and by the time Mr. Trump landed at Joint Base Andrews early the next morning, his critics were accusing him of sowing confusion on an issue of life or death.

The president immediately got on the phone with Alex M. Azar II, his secretary of health and human services. That call scared people, he shouted, referring to Ms. Messonnier’s warnings. Are we at the point that we will have to start closing schools? the president added, alarmed, according to an official who heard about the call.