Top federal health experts concluded by late February that the virus was likely to spread widely within the United States and that government officials would soon need to urge the public to embrace social distancing measures, such as avoiding crowds and staying home.

But Mr. Trump wanted to avoid disrupting the economy. So some of his health advisers, at Mr. Trump’s urging, told Americans at the end of February to continue to travel domestically and go on with their normal lives.

And they did. Millions moved across the country, cellphone data shows. Some unknowingly carried the virus with them.