text: First, the nanny quit. Dr. Kanna Vela had hired her to look after her young children, ages five and two and a half, while she worked the emergency departments at three different hospitals in the Greater Toronto Area (GTA). She used to run a busy family practice, too, for seven years, back before her second daughter was born. She loved it, but she quickly realized it left her with little time to see her children and made the difficult decision to move full-time to emergency medicine. Even her most chaotic shifts were regimented in strict slots during the evening, night, and early morning, leaving her precious hours with her kids while they were both still awake. Vela and her husband, who runs a software company, were making it work—until March and the pandemic and their nanny's understandable need to be at home with her own children. Vela wasn't sure it was even safe for her to be in her own house. She'd surely be working with COVID-19 patients, and the last thing she wanted was for her family to become some of them. So, along with her parents, she devised an elegant, heartbreaking solution: she'd move into their home, and they'd move into hers. Her daughters would be cared for and Vela would be alone.
  
  For weeks, she didn't see her family at all. Her husband read the girls their bedtime stories and tucked them in, while Vela read medical journals and attended hospital Zoom meetings. Eventually, it became unbearable. Though she knew there wasn't much science to back up her decision—there still wasn't very much science on the virus at all—she started making trips home on the third day of her three-day weekly break, so long as she felt well for the first two. There, she'd be sure to socially distance from her parents, playing a sort of musical chairs through the house, from the kitchen to the family room to the living room. Back at her makeshift home, she followed an elaborate sanitization routine—one she stuck with when, two months later, she finally moved back in with her family.
  
  Like many frontline staff treating COVID-19 patients, she avoided eating or even drinking water through most of her shift because the risk of removing her mask at the hospital was too high. At the end of her shift, she'd change at the hospital, then strip out of the clothes she wore home from work in the garage, and either tie them up or put them in the laundry right away. Then she'd run naked up to the shower. Nobody else could use her bathroom, just in case the steam from the hot water was infected and helped the virus aerosolize. Afterward, she'd use a Lysol wipe to clean anything she might have touched: her keys, her phone, doorknob handles. It took at least forty-five minutes, added on to the lengthy decontamination procedure she already did at the hospital. Sometimes, she wouldn't make it home until 3 or 4 a.m., barely able to drag her body to the shower. "You can easily forget things when you're tired and stressed," she said. "And then you spend the next day worrying, like, 'Did I miss a step? Will that mean someone at home gets sick?' "
  
  To help cope with the uncertainty, Vela focused on what she could control. Every healthcare worker in Canada had the perverse advantage of watching the pandemic unfold elsewhere first—the virus might have been sketched in question marks, but Canadian workers had more information than those in other countries did when they started. And they knew, if they were to have any chance, they needed PPE. Instead of waiting to run out of N95 masks, Vela and a few colleagues started a PPE drive. She got her husband to make a website. Another doctor reached out to CP24, the local news station, to talk about the drive. They urged chiropractors, dentists, family doctors, and any other medical office that was closed to donate their masks. "Yours will come," they urged them, "but we need them right now." Medical students volunteered to do inventory, and another doctor offered up their empty clinic to store the bounty. Word of mouth spread and people crafted face shields; Vela and her colleagues scoured the city for extra stock, for protection normally used in other, industrial fields. Fear threaded through those moments. Vela refused to let it paralyze her; action became an antidote.
  
  It all helped, but it didn't cure the frustration and the trauma of not quite knowing how to treat her patients. "We're trying to treat a disease that we barely know about," Vela said—a disease, she added, that had no treatment. "That can be very hopeless." Too much of her job was reduced to guesswork and supportive care. The constant surprise of helplessness gnawed at her. So too did watching patient after patient die alone, with nobody to hug them, to hold their hand, or to place a gentle, final kiss on their brow. That was true of all end-of-life patients during those months, whether they had COVID-19 or not. Sometimes a loved one could say goodbye through a tiny window slit. Later on, they could FaceTime through a donated iPad. Vela knew it wasn't enough. And she ached to know she couldn't give them, or their families, the dignified, full deaths they deserved. Sometimes she'd think about the already-emerging group of pandemic deniers and think about whether they'd still believe it was all a hoax if they could shadow her through the hospital, see the same sickness and grief that she did. If they could feel what it was like to try everything to save someone and still have it feel like throwing dice in Vegas.
  
  By April, the only thing that seemed certain about SARS-CoV-2 was that it did not act like any other coronavirus. "It can attack almost anything in the body with devastating consequences," said one U.S.-based doctor that month. "Its ferocity is breathtaking and humbling." A subsequent research paper published in the Lancet termed the stages of infection as "the four horsemen of a viral apocalypse." It deemed the first horseman a "sneaky virus." That virus, doctors and scientists knew, entered through the lining of the nose, where it found a lush expanse of cells that have a cell-surface receptor called angiotensin-converting enzyme 2, or ACE2, which is usually responsible for regulating blood pressure in the body. It also marks tissue potentially vulnerable to infection, and SARS-CoV-2 needs it to enter a cell. (In fact, any virus that infects animals needs the right cell receptor to infect—think of them as carrying, or perhaps stealing, the right key for the corresponding receptor lock.) After that, the virus begins its endless chain of reproduction and invasion. This can cause the more well-known symptoms of COVID-19: dry cough, sore throat, achy body, loss of smell and taste, fever. In the Bible, the first horseman is the Antichrist, riding in on a white horse, sowing disruption and readying the world for the other three—war, famine, death. If the body's immune system doesn't beat back the first horseman, things get similarly grim.
  
  The virus will continue down the weakened body, travelling the windpipe to attack the lungs. The lung's so-called respiratory tree ends in tiny air sacs called alveoli. A single layer of cells rich in ACE2 receptors lines each one—an irresistible home for the virus to ransack. This is when SARS-CoV-2 can turn deadly. As the immune system tries to defeat the virus, the resulting battle inhibits the healthy, normal flow of oxygen. Then come the classic pneumonia-like symptoms: pitching fever, coughing, a struggle to breathe. Sometimes, with medical intervention, such as a ventilator or even oxygen through simple nasal prongs, a patient can get better. Other times, most times, the battle between immune system and virus pushes a body toward rapid, baffling deterioration. An estimated 80 per cent of those infected with the virus remain asymptomatic, or only develop minor or moderate illness. They stay home, they isolate, and they get better. The other 20 per cent get sick enough to require hospital admission. And 5 per cent of those end up on ventilators, silently, unconsciously fighting for their lives in the ICU.
  
  In the first months of the pandemic, there seemed to be no way to tell which path a body would follow, or how to intercept the last horseman in any analogy: death. Compounding the unpredictability—the maddening senselessness of it all—was the virus's tendency to sometimes travel, and invade, any part of the body with ACE2 receptors. Usually, the chief mandate of a coronavirus is to reach the lungs. But SARS-CoV-2 can also target the kidneys, which have ample ACE2 receptors, and which, if infected, significantly heighten the risk of death. Sometimes it kept travelling down, to the intestines, causing diarrhea. Other times it targeted the liver, messing with enzyme levels. Some of the sickest patients developed pink eye. More rarely, the virus will make its way to the brain, which houses ACE2 receptors at its neural cortex and stem. It can invade them, but it can also suffocate nearby cells, sucking up needed oxygen and causing them to die. Neurological invasion can cause strokes, seizures, confusion, headaches, and delirium. At its deadliest, the virus seems insatiable, overtaking every part of the body it can. Some studies have suggested that pre-existing conditions, like asthma and diabetes, the size of the initial viral load, and age may provide clues as to how sick a person will get. Even with those scientific hints, however, concrete answers remained evasive.
  
  "We see these weird cases, and we can't just say, 'Oh, it's just elderly who are at high risk' or 'it's just patients who have other medical conditions,'" said Vela. "It would be easier if this disease followed rules." Every doctor or nurse who treated COVID-19 patients seemed to have at least one case they couldn't make sense of. For Vela, it was a man with a young family, who had returned from travel in March, and, as far as they could tell, had no major medical problems. He had come to the COVID-19 clinic after his cough and fever worsened. Staff at the clinic thought he didn't "look so good" and sent him to Vela's emergency department. She swabbed the man for the virus, did some blood work, and took some X-rays. It seemed likely he had the virus, but his vitals were stable and his X-rays looked okay. She sent him home to isolate, but also took his name and number so she could call him immediately with his results. There was no scientific reason for her worry—she just had a bad feeling. When his test came back positive the next day, she called him. He was still coughing, but he wasn't short of breath, the tell-tale sign of a worsening case. They decided he could stay at home and keep resting. But Vela couldn't shake her apprehension. She called him the next day, and the day after that. On the fourth call, she noticed a difference in his speech pattern, a shortness of breath. She told him to come back, paused, and said, "Actually, call 9-1-1." He needed an ambulance. She warned him that she'd call back to make sure he had; she didn't want to hear about any stereotypical male stubbornness.
  
  As soon as he arrived at the hospital, he was put on oxygen. His X-rays were covered with white splotches; black areas represent air. Shortly after, doctors intubated him and put him in the ICU, where he only got worse. Within a few days, they transferred him to Toronto General Hospital, which has an extra-corporeal membrane oxygenation (ECMO) machine. Considered to be the next level of therapy for COVID-19 when everything, even a ventilator, has failed, an ECMO machine is the most aggressive form of life support available. It pumps and oxygenates blood outside the body, then returns it—in this case, doing the work of the lungs when a patient's own have stopped working. It can perform miracles, but it just as often doesn't work. Vela followed the man's case from afar; he had small children at home and, painted in the broadest strokes, he reminded her of her husband, or even herself. He was relatively young, generally healthy, and they'd gone over and above with his care, calling him at home, pushing him to come in. Surely, if anybody could make it…The thought hurt too much to finish. But he didn't make it. When Vela found out he had died, she did the only thing she could now do: she cried.