The Dow Jones Industrial Average of 30 major American companies fell more than 4.5% on Friday, erasing all the gains it had made since Donald Trump became president in January 2017.
The drop helped to finish the worst week on Wall Street since 2008, with all three indexes down at least 12%.
The falls come as authorities tighten restrictions on activity in an effort to slow the spread of the coronavirus.
New York state on Friday ordered non-essential businesses to close.
Illinois also made a similar move, while California earlier mandated that its residents shelter in place.
The Dow lost more than 900 points to close at 19,173, while the wider S&P 500 dropped 4.3% to 2,304 and the Nasdaq lost 3.8% to 6,879.5.
They have now fallen more than 30% from their recent records.
Source: BBC News