Hall understood the need for evidence-based medicine as well as she did, he told her. But this was life and death. Under those circumstances, sometimes you don’t wait for standard procedure, he said. If it came to it, he was sure Cai would assume the risk. Hall suggested that he could provide Arad with a full translation of the guidelines, which had not yet been published in English; Arad, Hall said, took him up on the offer.

Lying in bed that night, Cai feared that he would close his eyes and never wake up — that he would slip away, essentially drowning in his sleep. He was being given oxygen, but even still, he saw his numbers trending downward — in the 80s. Concerned, he messaged a WeChat group that included his father and a doctor his father knew in Shanghai, who had been advising that Cai be put on a high-flow nasal oxygen cannula, a device that allows for a more intensive and stable delivery of oxygen into the lungs. Cai requested that treatment, but the nurses on duty said that they didn’t have the clearance to make that decision. Cai called Hall to ask for help in getting a doctor’s attention. Hall contacted a prominent local doctor, Henry Chen, who oversaw a sprawling network of community-health doctors in New York, in the hope that he could get in touch with someone at the hospital. Chen says he was told that because he did not have admitting privileges, he would not be put through.

Cai had never felt more alone. He repeatedly called for the nurse, and when she arrived, he spoke as harshly as he ever had to a fellow medical professional. “I am not going to sleep until I see a respiratory therapist,” he told her. He wanted closer monitoring; and he wanted the expert care of someone who could provide a higher level of oxygen dispersal. He dropped Chen’s name, even though he knew the name likely meant nothing to the nurse; he reminded the nurse that he was a physician assistant and could judge for himself his risk. Finally, at around midnight, a respiratory therapist arrived with a Venturi mask, providing a treatment that was not as powerful as high-flow but that still provided higher concentrations of oxygen than Cai had been getting. The therapist also took blood for a test that would assess Cai’s lung functioning.

Once he received the oxygen treatment, Cai allowed himself to drift off, though his dreams kept him on high alert. Sometimes he dreamed that he’d woken up — it was morning, and he was alive, which he knew because he was staring at the clock on the hospital wall. Sometimes he dreamed that the Chinese experts were telling him that they had seen the results of the blood test and that the numbers were not good. All night, he drifted between consciousness and slumber, his very dreams trying to make out whether he was going to live or die.

The morning of Sunday, March 8, Cai woke up. He knew he was alive. There was the clock. There was his phone with the photos of his friends and family, the beeping machinery above his head. And yet he was still afraid. He prayed to God; he prayed to Buddha. He bargained: He would save so many lives if only his own could be spared. He would stop working so hard so he could be a better father to his daughter. He read over and over the cards that friends had sent him, tangible objects from the outside world that let him know that he had not been forgotten. He continued to text with Huang, who by then was having his own anxieties. He was worried about his friend, but also about the new cases cropping up every day. “The reality was setting in,” he says. “We will become Wuhan, Milan.”

Later that day, around noon, Hall sat down in the study of his Long Island home to translate the Chinese medical guidelines. It was no small task, but he was not aware of any other translation, and he believed it was important. “No one had any experience here,” he told me. He opened a Microsoft Word document and started translating: the symptoms, the signs of mild cases, severe cases, the course of the disease, the methods of oxygen delivery, the recommendations for follow-up. Just before midnight, having worked for close to 12 straight hours, he sent it off to Arad. His sense of urgency extended beyond Cai’s case. If a health care professional like Cai could not be saved, he explained, his patients — many of whom speak almost no English — would feel they had no possibility of surviving the virus, should they catch it and experience complications.

Around the same time that Hall sat down to work in Long Island, Cai, lying in bed in his room in Hackensack, was surprised when a technician arrived in full P.P.E. He was going to get his second CT scan. Two hours later, when Dr. Balani came to see him with the results, Cai listened to her speak with some fascination and a little bit of fear. She seemed different. She sounded scared to him, but like someone trying hard to sound confident; he had the impression she had rehearsed what she was going to say. She was speaking more quickly than usual. And she was telling him that now it was time to take more aggressive measures. Eventually, Cai saw the scan himself. Instead of just that one white spot on one lung — something with the look of a dandelion gone to seed — there were dozens. The onslaught of the virus could be described as a toxic lava flow of infection that ravages the alveoli, the fragile, thin-membraned air sacs where gases are exchanged in each breath. It looked as if close to 40 percent of Cai’s lungs had succumbed in just five days.