New York is far more crowded than any other major city in the United States. It has 28,000 residents per square mile, while San Francisco, the next most jammed city, has 17,000, according to data from the U.S. Census Bureau.

All of those people, in such a small space, appear to have helped the virus spread rapidly through packed subway trains, busy playgrounds and hivelike apartment buildings, forming ever-widening circles of infections and making New York the nation’s epicenter of the outbreak…

The challenge facing New York and other tightly cramped cities around the United States can be seen by comparing the country’s largest city to its second biggest, Los Angeles.

As of Monday, there were more than 13,000 confirmed cases of coronavirus in New York and about 500 in Los Angeles. New York reported 125 deaths; Los Angeles reported seven.