U.S. equities posted their biggest weekly decline since August as growing concerns about the spread of the deadly coronavirus outweighed blowout earnings reports from some of the biggest technology companies.

The S&P 500 Index sank 2.1 percent, extending the drop from its Jan. 17 record to 3.1 percent. The Dow Jones industrial average retreated 2.5 percent, with 24 of its 30 members ending the week lower. The Nasdaq Composite Index fell 1.8 percent.

This week’s drop came as the number of cases of the coronavirus that originated in China soared. Investors have grown increasingly worried that the spread of the illness will crimp global economic growth. Warwick McKibbin, a professor of economics at the Australian National University in Canberra, said the global cost could be three or four times that of the 2003 SARS outbreak that sapped the world’s economy by $40 billion. That damped the enthusiasm behind strong quarterly earnings results from companies including Apple, Microsoft and Amazon. (Jeff Bezos, who founded Amazon, also owns The Washington Post.)

Nine of the main 11 S&P 500 industry groups fell for the week. Energy stocks led the decline, sinking 5.7 percent.

The U.S. Treasury will sell 13-week bills and 26-week bills on Tuesday. It will also sell four-week bills and eight-week bills on Thursday.