U.S. stocks plunged Thursday in their worst day since the 1987 crash. The Dow Jones Industrial Average fell 10%, and the S&P 500 and Nasdaq tumbled nearly as much to join the Dow in a bear market.

The furious falls in share prices on rising fears of a global slowdown due to the rapid spread of coronavirus occurred despite a $1.5 trillion intervention in short-term funding markets by the Federal Reserve.

The...