March 9 was supposed to be the start of a new routine for JPMorgan Chase & Co. employees. With coronavirus spreading, the bank had told the staff in its stock-trading operation to head to three separate sites around New York City.

Hours before the workday began, with global markets plunging, technology at the sites wasn’t ready. JPMorgan top brass reversed the order and told many traders to report for duty, as usual, to the firm’s Manhattan headquarters, employees said.

An employee who wasn’t feeling well came to the office. JPMorgan traded more shares that Monday than any day in the bank’s history. The sick employee turned out to have Covid-19, and over the past three weeks, about 20 employees on a single floor at the bank’s headquarters have tested positive for the virus, with another 65 quarantined as a result.

Wall Street is used to making tough choices in seconds, but the coronavirus pandemic has added a dimension of life or death. Amid the wildest trading conditions in more than a decade, banks are loath to fully allow the thousands of traders and salespeople who keep the markets humming to work from home. Setups in home offices lack the multibillion-dollar technology infrastructure of the trading floor. Even slight delays in speed could cost money.

Wall Street trading has been deemed an “essential service” by New York authorities, and though the New York Stock Exchange has shut its floor, the major banks continue to have some employees report to work.