Still, there was little for him to do. “We knew it was best to watch the mayhem from the sidelines.”

Tuesday, Feb. 25, 12 p.m.

A selling frenzy follows a scary health warning.

Jeb Breece glanced at the nearby TV, tuned as always to CNBC. The volume was off, but it took only a moment for Mr. Breece, a principal at the New York money-management firm Spears Abacus, to glean what was happening. After Monday’s 3.4 percent decline, the markets had opened ugly and were getting worse.

The catalyst, as far as Mr. Breece could tell, was a warning by a federal health official that the coronavirus was coming to America; it was a question of when, not if. For a market on edge, this was enough to set off a fresh wave of frenzied selling.

Mr. Breece had been thinking for weeks that the coronavirus was going to take a bite out of economic growth and corporate profits. In mid-February the Chinese e-commerce giant Alibaba — a stock he owns — had warned that the virus would cause its sales to decline in the first quarter of 2020. That was a clear sign to Mr. Breece that the Chinese economy — the world’s second-largest — was in trouble.

Mr. Breece had started selling shares of companies whose fortunes are tied to the short-term ups and downs of the economy. Among other moves, he’d gotten rid of all of his shares of Schlumberger, the giant oil-services company.

But on early Tuesday afternoon, he was a bit surprised to see investors reacting with alarm to a federal official’s borderline-obvious prediction that America would not be immune to a fast-spreading virus. “I said, ‘Is that new news?’” Mr. Breece recalled. “Of course, this is bad.” But the selling pressure kept building, as the rest of the world focused on what Mr. Breece had already anticipated.

Thursday, Feb. 27, 9:45 a.m.

He’s on vacation, watching his phone.

The aircraft doors were closed, the plane was preparing to depart from Newark to Orlando, Fla., and Douglas Boneparth was anxious.