BEIJING (NYTIMES) - The young mothers didn't tell their children they had the coronavirus. Mama was working hard, they said, to save sick people.

Instead, nurse Deng Danjing and Dr Xia Sisi were fighting for their lives in the hospitals where they worked, weak from fever and gasping for breath. Within a matter of weeks, they had gone from healthy medical professionals on the front lines of the epidemic in Wuhan, China, to coronavirus patients in critical condition.

The world is still struggling to fully understand the new virus, its symptoms, spread and sources. For some, it can feel like a common cold. For others, it is a deadly infection that ravages the lungs and pushes the immune system into overdrive, destroying even healthy cells. The difference between life and death can depend on the patient's health, age and access to care - although not always.

The virus has infected more than 132,000 globally. The vast majority of cases have been mild, with limited symptoms. But the virus's progression can be quick, at which point the chances of survival plummet. Around 68,000 people have recovered, while nearly 5,000 have died.

The fates of Ms Deng and Dr Xia reflect the unpredictable nature of a virus that affects everyone differently, at times defying statistical averages and scientific research.

As the new year opened in China, the women were leading remarkably similar lives. Both were 29 years old. Both were married, each with a young child on whom she doted.

Ms Deng, a nurse, had worked for three years at Wuhan No. 7 Hospital, in the city where she grew up and where the coronavirus pandemic began. Her mother was a nurse there, too, and in their free time they watched movies or shopped together. Ms Deng's favourite activity was playing with her two pet kittens, Fat Tiger and Little White, the second of which she had rescued just three months before falling sick.

Dr Xia, a gastroenterologist, also came from a family of medical professionals. As a young child, she had accompanied her mother, a nurse, to work. She joined the Union Jiangbei Hospital of Wuhan in 2015 and was the youngest doctor in her department. Her colleagues called her "Little Sisi" or "Little Sweetie" because she always had a smile for them. She loved Sichuan hot pot, a dish famous for its numbingly spicy broth.

When a mysterious new virus struck the city, the women began working long hours, treating a seemingly endless flood of patients. They took precautions to protect themselves. But they succumbed to the infection, the highly contagious virus burrowing deep into their lungs, causing fever and pneumonia. In the hospital, each took a turn for the worse.

One recovered. One did not.

SYMPTOMS: Onset of Virus and Hospitalisation

The symptoms came on suddenly.

Dr Xia had ended her night shift Jan 14 when she was called back to attend to a patient - a 76-year-old man with suspected coronavirus. She dropped in frequently to check in on him.

Five days later, she started feeling unwell. Exhausted, she took a two-hour nap at home, then checked her temperature: It was 38.8 deg C. Her chest felt tight.

A few weeks later, in early February, Ms Deng, the nurse, was preparing to eat dinner at the hospital office when the sight of food left her nauseated. She brushed the feeling aside, figuring she was worn out by work. She had spent the beginning of the outbreak visiting the families of confirmed patients and teaching them to disinfect their homes.

After forcing down some food, she went home to shower, and then, feeling groggy, took a nap. When she woke up, her temperature was 38 deg C.

Fever is the most common symptom of the coronavirus, seen in nearly 90 per cent of patients. About a fifth of people experience shortness of breath, often including a cough and congestion. Many also feel fatigued.

Both women rushed to see doctors. Chest scans showed damage to their lungs, a telltale sign of the coronavirus that is present in at least 85 per cent of patients, according to one study.

In particular, Ms Deng's CT scan showed what the doctor called ground-glass opacities on her lower right lung - hazy spots that indicated fluid or inflammation around her airways.

The hospital had no space, so she checked into a hotel to avoid infecting her husband and five-year-old daughter. She sweated through the night. At one point, her calf twitched. In the morning, she was admitted to the hospital. Her throat was swabbed for a genetic test, which confirmed she had the coronavirus.

Her room in a newly opened staff ward was small, with two cots and a number assigned to each one. Ms Deng was in bed 28. Her roommate was a colleague who had also been diagnosed with the virus.

At Jiangbei Hospital, 29km away, Dr Xia was struggling to breathe. She was placed in an isolation ward and treated by doctors and nurses who wore protective suits and safety goggles. The room was cold.

TREATMENT: Day 1, Hospitalisation Begins

When Ms Deng checked into the hospital, she tried to stay upbeat. She texted her husband, urging him to wear a mask even at home and to clean all their bowls and chopsticks with boiling water or throw them out.

Her husband sent a photograph of one of their cats at home. "Waiting for you to come back," he said.

"I think it'll take 10 days, half a month," she replied. "Take care of yourself."

There is no known cure for Covid-19, the official name for the disease caused by the new coronavirus. So doctors rely on a cocktail of other medicines, mostly antiviral drugs, to alleviate the symptoms.

Ms Deng's doctor prescribed a regimen of arbidol, an antiviral medicine used to treat the flu in Russia and China; Tamiflu, another flu medicine more popular internationally; and Kaletra, an HIV medicine thought to block the replication of the virus. Ms Deng was taking at least 12 pills a day as well as traditional Chinese medicine.

Despite her optimism, she grew weaker. Her mother delivered home-cooked food outside the ward, but she had no appetite. A nurse had to come at 8.30am each day to hook her up to an intravenous drip with nutrients to feed her. Another drip pumped antibodies into her bloodstream, and still another antiviral medicine.

Dr Xia, too, was severely ill, but appeared to be slowly fighting the infection. Her fever had subsided after a few days, and she began to breathe more easily after being attached to a ventilator.

In early February, she asked her husband, Wu Shilei, also a doctor, whether he thought she could get off oxygen therapy soon.

"Take it easy. Don't be too anxious," he replied on WeChat. He told her that the ventilator could possibly be removed by the following week.

"I keep on thinking about getting better soon," she responded.

There was reason to believe she was on the mend. After all, most coronavirus patients recover.

Later, Dr Xia tested negative twice for the coronavirus. She told her mother she expected to be discharged on Feb 8.

DECLINE: Days 4 to 15 After Hospitalisation

By Ms Deng's fourth day in the hospital, she could no longer pretend to be cheerful. She was vomiting, having diarrhoea and relentlessly shivering.

Her fever jumped to 38.5 deg C. Early in the morning on Feb 5, she woke from a fitful sleep to find the medicine had done nothing to lower her temperature. She cried. She said she was classified as critically ill.

China defines a critically ill patient as someone with respiratory failure, shock or organ failure. Around 5 per cent of infected patients became critical in China, according to one of the largest studies to date of coronavirus cases. Of those, 49 per cent died. Those rates may eventually change once more cases are examined around the world.

While Dr Xia appeared to be recovering, she was still terrified of dying. Testing can be faulty, and negative results don't necessarily mean patients are in the clear.

She asked her mother for a promise: Could her parents look after her two-year-old son if she didn't make it?

Hoping to dispel her anxiety with humour, her mother, Jiang Wenyan, chided her: "He's your own son. Don't you want to raise him yourself?"

Dr Xia also worried about her husband. Over video chat, she urged him to put on protective equipment at the hospital where he worked. "She said she would wait for me to return safely," he said, "and go to the front line again with me when she recovered."

Then came the call. Dr Xia's condition had suddenly deteriorated. In the early hours of Feb 7, her husband rushed to the emergency room. Her heart had stopped.

RECOVERY: DAY 17 After Hospitalisation

In most cases, the body repairs itself. The immune system produces enough antibodies to clear the virus, and the patient recovers.

By the end of Ms Deng's first week in the hospital, her fever had receded. She could eat the food her mother delivered. On Feb 10, as her appetite returned, she looked up photos of meat skewers online and posted them wishfully to social media.

On Feb 15, her throat swab came back negative for the virus. Three days later, she tested negative again. She could go home.

Ms Deng met her mother briefly at the hospital's entrance. Then, because Wuhan remained locked down, without taxis or public transportation, she walked home alone.

"I felt like a little bird," she recalled. "My freedom had been returned to me."

The Chinese government has urged recovered patients to donate plasma, which experts say contains antibodies that could be used to treat the sick. Ms Deng contacted a local blood bank soon after getting home.

She plans to go back to work as soon as the hospital allows it.

"It was the nation that saved me," she said. "And I think I can pay it back to the nation."

DEATH: Day 35 After Hospitalisation

It was sometime after 3am on Feb 7 when Dr Xia was rushed to intensive care. Doctors first intubated her. Then, the president of the hospital frantically summoned several experts from around the city, including Dr Peng Zhiyong, head of the department of critical care at Zhongnan Hospital.

They called every major hospital in Wuhan to borrow an extra-corporeal membrane oxygenation, or ECMO, machine to do the work of her heart and lungs.

Dr Xia's heart started beating again. But the infection in her lungs was too severe, and they failed. Her brain was starved of oxygen, causing irreversible damage. Soon, her kidneys shut down, and doctors had to put her on round-the-clock dialysis.

"The brain acts as the control centre," Dr Peng said. "She couldn't command her other organs, so those organs would fail. It was only a matter of time." Dr Xia slipped into a coma and died on Feb 23.

Dr Peng remains baffled about why Dr Xia died after she seemed to improve.

Her immune system, like that of many health workers, may have been compromised by constant exposure to sickness. Perhaps she suffered from what experts call a "cytokine storm", in which the immune system's reaction to a new virus engulfs the lungs with white blood cells and fluid. Perhaps she died because her organs were starved of oxygen.

Back at Dr Xia's home, her son, Jiabao - which means priceless treasure - still thinks his mother is working. When the phone rings, he tries to grab it from his grandmother's hands, shouting, "Mama, Mama". Her husband, Wu, doesn't know what to tell Jiabao. He hasn't come to terms with her death himself. They had met in medical school and were each other's first loves. They had planned to grow old together.

"I loved her very much," he said. "She's gone now. I don't know what to do in the future. I can only hold on."