New York City is barreling toward crisis at an alarming rate, with the rapid spread of the coronavirus on track to crush intensive-care units in as little as two weeks, Gov. Andrew Cuomo said on Tuesday.

“We are not slowing it; it is accelerating on its own,” Cuomo said, adding that the number of COVID-19 infections is now doubling every three days.

In New York City, whose urban density has helped made it the epicenter of the outbreak in the U.S., there were 14,776 confirmed cases and 131 deaths as of Tuesday. That’s more than six times as many cases as hard-hit Washington state and more than eight times the number of cases in all of California.

“We were looking at a freight train coming across the country. We are now looking at a bullet train,” said Cuomo, citing one of the scientists charting the disease’s progression.

Throughout the five boroughs, subway cars, once packed toe-to-heel by straphangers, are running nearly empty; restaurants, bars, flower shops and museums have shuttered; at some parks, lines chalked on the pavement measure 6 feet, the distance people must keep between one another. And still the rate of infection is quickening.

A severe health-care shortage in New York City is not lost on the White House. In a speech on Monday, Dr. Deborah Birx, a leading coordinator of the U.S. coronavirus task force, said the virus’s so-called attack rate, or the number of people infected compared with the general population, in the New York metropolitan area is close to 1 in 1,000. The attack rate is even higher in New York City’s five boroughs, where the latest data show that 1 in every 600 people is infected.

“So, to all of my friends and colleagues in New York, this is the group that needs to absolutely social distance and self-isolate at this time,” Birx said.

At a news conference late Tuesday at the White House, Vice President Mike Pence, who joins Birx in heading up the White House task force, called on people who have recently been in and left New York City to self-isolate for 14 days.

Florida on Tuesday mandated that all travelers arriving from New York state, as well as New Jersey and Connecticut, self-quarantine for two weeks.

New York City’s density is such that a quarter of a million people would funnel through a single subway stop at Times Square on an average weekday before the pandemic, and most people live in apartments where contact among neighbors in elevators, stairwells and hallways is all but inevitable — even when social distancing.

“The density of the population is reason No. 1, No. 2 and No. 3 — it’s just been so easy for it to spread,” said Dr. Josh Sharfstein, vice dean for public health practice and community engagement at the Johns Hopkins Bloomberg School of Public Health.

Manhattan is the most densely populated county in America, with around 67,000 people per square mile, and three other boroughs, Brooklyn, the Bronx and Queens, are respectively the second, third and fourth densest, illustrating why the coronavirus has been able to expand its footprint so quickly as compared with other cities experiencing outbreaks. In Los Angeles County, for example, density is closer to 2,500 people per square mile, making it about half as densely populated as Cook County in Illinois, where Chicago is located.

New York City also has some of the most robust data about the prevalence of the disease, as the state ramped up testing capability and set up a number of drive-through test sites. The labs are testing more people per capita than anywhere else in the world, Cuomo said Tuesday.

But expansive testing is not the reason New York City has so many confirmed cases.

“It only takes a quick second to look at the health-care system in New York and the number of very sick patients to know that it’s the epicenter,” Johns Hopkins’s Sharfstein said.

There’s another aspect to population density that made poorer New York City neighborhoods particularly susceptible, said Jessica Justman, an associate professor of medicine in epidemiology at Columbia Mailman School of Public Health.

“It is also about the average number of people living in a household. People living alone in an apartment will be able to practice social distancing more easily than people who live with a large family, or who are sharing space with many others,” Justman said.

Household density explains why Queens is the hardest hit borough in the city, as it has a relatively larger population of low-income immigrant families living in greater numbers in smaller spaces, she said. “It makes sense that the epidemic would concentrate in areas where many people occupy the same household,” she said.

Queens accounts for 30% of all cases in the city, with 4,364 people having contracted the coronavirus as of Tuesday, compared with 2,887 in the more affluent Manhattan.

It also explains why the city continues to see cases rise despite stay-at-home orders, with the virus spreading among family members.

Beckie Strum writes for the Barron’s Group from New York.

MarketWatch coronavirus update: 407,405 diagnosed COVID-19 cases, 18,227 deaths; Italy shows glimmer of hope; New York remains the U.S. epicenter