New York State alone accounted for more than 30,000 cases and close to 300 deaths, most of them in New York City.
Governor Andrew Cuomo, again pleading for help in dealing with the coming onslaught, attributed the cluster to the city's role as a gateway to international travellers and the sheer density of its population, with 8.6 million people sharing trains, lifts, apartment buildings and offices.
"Our closeness makes us vulnerable," he said. "But it's true that your greatest weakness is also your greatest strength. And our closeness is what makes us who we are. That is what New York is."
More than 440,000 people worldwide have been infected overall, by Johns Hopkins' count. More than 100,000 have recovered.
Other American states braced for a version of New York's nightmare, with fears over public events held in the weeks before the virus exploded.
A month after Mardi Gras in and around New Orleans, Louisiana is seeing a ballooning number of cases and now has the third-highest rate per capita in the US, according to the governor. Sixty-five Louisiana residents have died, and the virus has been confirmed in three-quarters of the state's 64 parishes.
- AP