As everyone knows, New York City is the main center of COVID-19 infection and fatality in the U.S., with something like 40% of all American deaths. The reasons why shouldn’t be a mystery.

When the Black Death struck Europe in the 14th Century, people had no idea what caused the disease, and all of their theories were wrong. But experience showed, as was the case in later plagues, that the countryside was safer than the city. Most people who could decamped from cities to the country.

Essentially the same thing is true today. Densely populated areas create more contacts with others and more opportunities for a virus to spread. And New York City, with more than 28,000 people per square mile, is by far the most densely populated major US city. San Francisco is second at around 18,500, and cities like Chicago, Philadelphia and Washington are in the 11,000 to 12,000 range. So one would expect COVID-19 to spread more rapidly in New York than anywhere else. It is noteworthy, too, that the New Jersey cities across the water from Manhattan are also very densely populated, about as densely as New York, and are very much part of the metro area COVID picture.

There is a second factor that I suspect is also very important: New York’s reliance on mass transit. New York’s mass transit system–mostly its subways–is, by a huge margin, the most relied-upon public transportation system in the U.S. It carries a remarkable 2,275,000,000 riders per year. The second most-used public transit system is the Washington Metro, which carries only 238,000,000 riders per year, just over 10% of New York’s total. Chicago’s “L” carries fewer than 10% as many riders as New York’s subways. Public transportation, especially a subway, is no doubt the place where one comes into contact with the largest number of random strangers, and is foreseeably a common site of transmission.

Another point is worth mentioning. Why don’t we think of New York as a chronically sick place? Because people build up immunity to diseases that have been around for a while, and the more people you are exposed to, the more likely you are to be immune. But when a new disease like COVID-19 comes along, a virus to which no one has built up immunity, the population density and public transportation factors come to the fore and New York becomes a very sick city, compared to anywhere else.