Then came news that factory activity in China — one of the world’s largest economies — fell 13.5 percent last month compared with February of last year. Investment in China fell by roughly 25 percent. And one of the first bits of data for American economic activity in March, a gauge of manufacturing activity in New York State, showed a record one-month plunge in the measure, which fell to its lowest level since 2009.

“Unfortunately, this is the new reality. This report is a harbinger of what is to come,” wrote economic analysts with the investment bank Jefferies in New York.

The Trump administration released new public guidelines on Monday to slow the spread of the coronavirus, including closing schools and avoiding groups of more than 10 people, discretionary travel, bars, restaurants and food courts.

The S&P 500 fell 12 percent, its biggest drop since the coronavirus outbreak began to roil markets in the United States last month — and its worst daily decline since October 1987, when stocks plunged about 20 percent in what came to be known as Black Monday. For the technology heavy Nasdaq, the drop was its worst on record.

Energy prices also slid sharply as investors factored in significant slowdowns in economic activity.