Gov. Andrew Cuomo of New York was blunt about the rationale behind this time of quarantine.

“There is a density level in NYC that is destructive,” he tweeted Sunday, after similar comments at one of his daily press briefings. He’d seen New Yorkers out in parks together, behaving as if this were a normal sunny spring weekend, and he was dismayed. Togetherness itself could now be deadly.

“It has to stop and it has to stop now,” he tweeted. “NYC must develop an immediate plan to reduce density.”

This has been an especially painful realization in major cities: The very thing that makes cities remarkable — the proximity of so many people to one another — is now making them susceptible in a pandemic. Density, suddenly, is bad for our health. And we are trying everything we can think of to dismantle it.

Special grocery store hours for older people — those are about reducing density. Closed schools and dispersed children — the same.