The next day, board members — many of them Republican donors — got another taste of government uncertainty from Larry Kudlow, the director of the National Economic Council. Hours after he had boasted on CNBC that the virus was contained in the United States and “it’s pretty close to airtight,” Mr. Kudlow delivered a more ambiguous private message. He asserted that the virus was “contained in the U.S., to date, but now we just don’t know,” according to a document describing the sessions obtained by The New York Times.

The document, written by a hedge fund consultant who attended the three-day gathering of Hoover’s board, was stark. “What struck me,” the consultant wrote, was that nearly every official he heard from raised the virus “as a point of concern, totally unprovoked.”

The consultant’s assessment quickly spread through parts of the investment world. U.S. stocks were already spiraling because of a warning from a federal public health official that the virus was likely to spread, but traders spotted the immediate significance: The president’s aides appeared to be giving wealthy party donors an early warning of a potentially impactful contagion at a time when Mr. Trump was publicly insisting that the threat was nonexistent.