Encouragingly, others have taken the lead. The San Francisco district attorney, Chesa Boudin, together with the public defender, Manohar Raju, were the first to take proactive steps to release as many people as safely possible who are at heightened risk from coronavirus. Mr. Boudin directed his prosecutors not to oppose release motions for misdemeanor or nonviolent felony pretrial detainees where the person poses no threat to public safety.

“We are trying to absorb information from countries who have experienced the Covid-19 pandemic before us,” said Dr. Alysse Wurcel, an infectious diseases physician at Tufts Medical Center and at six county jails in eastern Massachusetts. “But since the United States has the highest incarceration rate in the world, it is difficult to extrapolate the potential impact.”

American officials can learn from the harrowing story of South Korea’s Daenam Hospital. In late February, South Korea had already reported more than 3,150 confirmed cases, and of these, 101 were from patients in the Daenam psychiatric ward. Seven of these patients have now died. All but two patients in the ward contracted Covid-19. The ward was put on lockdown, in an attempt to confine the spread of the virus. Instead, the lockdown issued was a death sentence to many inside.

Across the United States, activists for prisoners’ rights have repeatedly requested plans to protect against an outbreak in prisons. Still, only a few jurisdictions have released plans. Some make good sense — one from the New York City Department of Correction includes screening people for flulike symptoms before placing them in group holding cells, and sending people who have flulike symptoms to a communicable diseases unit for treatment.

But those steps do not go far enough, nor do they affirmatively indicate an understanding of the ways this virus spreads: Will the incarcerated laborers now creating “NYS Clean,” the New York State government-manufactured hand sanitizer, be wearing N95 masks and gloves? The plan indicates that people incarcerated in dormitories on Rikers Island are being asked to sleep head to toe and three feet apart in the bunks, as if this short distance could prevent the spread of the virus if it’s present. It won’t.

There are yet more reasons to be concerned. With about 40 percent of incarcerated people suffering from a chronic health condition, the overall health profile of people in jails and prisons is abysmal. And the older prison population is among the most vulnerable to severe complications from Covid-19. There are 274,000 people aged 50 or older in state and federal prisons in the United States. If this group were separated from the rest of the U.S. prison population, they would be the seventh-largest prison system in the world.

Aging people who are released after serving long sentences have a recidivism rate close to zero. Governors and other public officials should consider a one-time review of all elderly or infirm people in prisons, providing immediate medical furloughs or compassionate release to as many of them as possible.