In the most severe scenario, where the virus spreads more widely across countries and sectors of the economy, growth slows to a near halt in the United States for several quarters this year, leaving 2020 with a 0.5 percent growth rate over all. “The risks are clearly skewed to the downside until the outbreak is contained,” researchers at Goldman Sachs said in a note this week.

There was clear evidence in recent days of the economic fallout. Toll Brothers, the luxury home builder, said some home sales to Chinese buyers had been postponed and shipments of fixtures from China delayed. The shoemaker Steve Madden said some shipments would be delayed by three weeks as its Chinese factories struggle to operate with fewer workers.

Perhaps even more troubling were signs that American consumers, who drive the economy, were becoming increasingly uneasy.

On Amazon, popular brands of hand sanitizers like Purell were largely unavailable. What was available was coming from third-party sellers at higher prices. On Friday morning, one pack of two 12-fluid-ounce bottles of Purell was being offered by a third-party seller for $49.99.

Even the parent company of Corona, the beer brand, has seen its shares drop more than the broader market, which some have attributed to its having the same name as the virus.

Such fears do not bode well for a modern economy and stock market that depend on optimism and a willingness to spend. As recently as nine days ago, that optimism helped drive up the stock market to a new high.

But in the last week, the rosy outlook that corporate profits would keep growing has been replaced by panic, said Richard Sylla, professor emeritus at New York University’s Stern School of Business, who has studied stock market shocks through history.