As the crisis has escalated, Eric Toner has tried to work out how many more breathing machines are available nationally. There is a federal stockpile of ventilators, for use in emergencies, which Anthony Fauci, the director of the National Institute for Allergy and Infectious Diseases, has put at twelve thousand and seven hundred. When President Donald Trump told a group of governors, on a conference call on Monday, “Respirators, ventilators, all the equipment—try getting it yourselves,” he was presumably holding this stockpile in reserve. Anesthesia machines, normally used in surgeries, can provide some similar functions to ventilators, though using one for a patient with pneumonia would mean that it could not be used for a surgery. Transport ventilators often used in ambulances could, in theory, be repurposed to be useful for some less severe I.C.U. patients. “A very ballpark number: maybe you could get up to a hundred and fifty thousand ventilators,” Toner told me on Monday. “That would mean cancelling almost all surgeries and all procedures, and taking very dramatic and drastic steps that would create a great deal of difficulty. I think that is the absolute limit, and I’m not sure we could get to that number.”

Toner, Li, and their co-authors had estimated that, in cities with major COVID-19 outbreaks, between 2.1 and 4.9 ventilators would be needed per every ten thousand people. I asked Toner what those numbers would mean for American cities. “Those cities and hospitals that are on the low end of that spectrum, they will be extraordinarily stressed, like they’ve never been stressed before,” Toner said. “On the upper end, that would be something like they’ve never imagined before, where, no matter what we do, we don’t have enough mechanical ventilators for everyone who needs them, and we will need to make allocation decisions—rationing decisions.”

So far, the pressure of this outbreak has been on public-health systems and administrators to “dig under the couch cushions,” as Hick put it, to try to increase their capacity as best they can. But some doctors are also beginning to think about what might happen if there is not enough capacity. In Italy, as pressures stemming from the coronavirus rose, guidance published by that country’s national association of critical-care doctors suggested that, if rationing was needed, ventilators should be reserved for patients with “the greatest life expectancy.” The practical problems—finding more breathing machines—lead directly to ethical ones about what to do if there are not enough ventilators and care has to be rationed, with some patients chosen over others. Those decisions would belong not to the system, in some general way, but to doctors, and in particular the pulmonologists running the I.C.U.s.

My wife, Heather Parsons, is an oncologist who works at the Dana-Farber Cancer Institute, on the western edge of Boston. Several other large hospitals (Brigham and Women’s, Beth Israel Deaconess, Boston Children’s) are situated within a few blocks, as is Harvard Medical School, with which all of these institutions are affiliated. We live about fifteen blocks away from the medical campus, in the Coolidge Corner neighborhood of Brookline, in what must be one of the world’s densest concentrations of doctors and medical scientists, talent drawn from all over the planet. They are impressive people, and they are perhaps especially impressive to one another—no one wants to be the weak link. In my experience, doctors think about their patients but also about their fellow-doctors and nurses and medical staff—about the vast, collaborative, intricately organized enterprise of professionals of which they are part. “Everyone tries so hard,” Heather told me once, when she was in medical school and starting to work in hospitals, and she has said it hundreds of times in the years since. Everyone is always trying so hard.

This past week, everyone has been trying even harder. Each time I dropped our son off at the medical-faculty day care, it was awash in weighty new rumors. Certain surgeries, less acute ones, were said to be postponed. Some doctors were said to be quarantined at home for fourteen days, uneasy and anxious at being sidelined. Only the Massachusetts State Public Health Lab was certified to test for COVID-19, which made it all but impossible to have a suspected case confirmed. Doctors were having to make decisions about what precautions to take without knowing for sure who had the virus. Even the mundane workings of the hospital, such as the waiting rooms and elevators, carried new risks, and so decisions that had been easy—say, about whether to have a patient in for a follow-up—became hard. Many of Heather’s patients, people with breast cancer, are on immunosuppressive drugs, and are particularly vulnerable. These decisions had little to do with the limits on ventilator capacity, but they shared the same sharpening realization of trade-offs.

A friend of mine suggested that I meet a colleague of his, Emmy Rubin, an attending intensive-care pulmonologist at Massachusetts General and the co-chair of the hospital’s Optimum Care Committee, which she described as the “ethics committee.” We arranged to meet on Friday afternoon, in a pub across the street from Mass General, and though I showed up fifteen minutes early she was already there—a small woman in a green raincoat, with short brown hair, Tina Fey glasses, and an intense and worried look—with the punctuality I’ve come to expect from doctors. Rubin has been involved in planning for the response to the outbreak. When I met her, she was also preparing for a scheduled two-week rotation running one of Mass General’s medical I.C.U.s, beginning on Wednesday, and was trying to weigh what she would do if the I.C.U.s became overwhelmed. Rubin told me that, in some personal ways, she was “totally unprepared.” She said, “Should I be, like, writing letters to everyone I care about? I mean, I guess it can’t hurt, but on the other hand am I realistically going to do that, like, this weekend?”

In the Mass General I.C.U.s—the best-resourced units in one of the very best-resourced hospitals in the best-resourced country in the history of the world—the general American assumption of infinite possibility reaches something like an apogee. “Our culture here in the United States is that people who want critical care generally receive it,” Rubin said. “So the idea of not being able to offer it, or saying there are certain people we cannot offer it to, is very new, and a huge departure.” As the stories of strict age limits that were being recommended for ventilator allocation in Italy spread, Rubin had been thinking through the ways in which various ethical frameworks might be applied to rationing care. A strict age cutoff had the advantage of taking all subjectivity out of a doctor’s decision, she said, but it also seemed a little hard to defend, because not all people are equally healthy or equally likely to live a long life. She also raised the idea that some people, based on their professions, might receive preferential treatment, such as “health-care workers or other people who could help save lives or provide a valuable social good that contributes to solving these problems in the future.”

In the abstract, it is easy enough to say that a healthier patient with longer to live ought to be prioritized over a sick one. But when a doctor is confronted with two charts of people whom she has never met, it is not easy to quickly make a call. Within the community of medical ethicists, Rubin said, there had been some efforts to define categories of patients—those with malignant metastatic cancer, for instance—who might receive lower priority. But even that seemed too crude: with modern therapies, Rubin pointed out, you could have patients with metastatic breast cancer who would live for fifteen years. “As you try to come up with categories, you really recognize the limitations of those categories and the difficulty of lumping people together who are not all the same,” she said. “Anything but a strict cutoff is going to be extremely subjective.” But extremely subjective measures are still, she said, “probably the best tool.”