By Thursday afternoon, texts and emails circulating among New York’s professional class warned that perhaps it was time to leave. The city was going into lockdown, or at least that’s what people were saying — probably by Friday morning. No one really knew this but now, you did.

The rumor was false but that didn’t slow it down. Officials weren’t going to tell you because they wanted to prevent a panicked exodus from New York that would recreate the early scenes in “War of the Worlds.” People took from this that they should begin heading toward I-95, that they should get on the Long Island Expressway.

The notes shared a familiar tone. “A friend just alerted me that her friend who works in the emergency management team at the N.Y.P.D. plans to put containment actions in place this weekend,” read one.

They varied only in terms of the information’s supposed origins: the friend of a friend at City Hall; the friend with a client high up in the medical community; the friend who knew someone who had just had lunch with Emma Bloomberg. Indeed, Michael Bloomberg’s daughter became a recurrent character in what was now the beginning of Covid-19’s urban legend phase.