Adults are not nearly so fortunate. Few grown humans can normally survive a fall of much more than twenty-five or thirty feet, though there have been some notable exceptions—none more memorable perhaps than that of a British airman in World War II named Nicholas Alkemade.

  In the late winter of 1944, while on a bombing run over Germany, Flight Sergeant Alkemade, the tail gunner on a British Lancaster bomber, found himself in a literally tight spot when his plane was hit by enemy flak and quickly filled with smoke and flames. Tail gunners on Lancasters couldn’t wear parachutes because the space in which they operated was too confined, and by the time Alkemade managed to haul himself out of his turret and reach for his parachute, he found it was on fire and beyond salvation. He decided to leap from the plane anyway rather than perish horribly in flames, so he hauled open a hatch and tumbled out into the night.

  He was three miles above the ground and falling at 120 miles per hour. “It was very quiet,” Alkemade recalled years later, “the only sound being the drumming of aircraft engines in the distance, and no sensation of falling at all. I felt suspended in space.” Rather to his surprise, he found himself to be strangely composed and at peace. He was sorry to die, of course, but accepted it philosophically, as something that happened to airmen sometimes. The experience was so surreal and dreamy that Alkemade was never certain afterward whether he lost consciousness, but he was certainly jerked back to reality when he crashed through the branches of some lofty pine trees and landed with a resounding thud in a snowbank, in a sitting position. He had somehow lost both his boots, and had a sore knee and some minor abrasions, but otherwise was quite unharmed.

  Alkemade’s survival adventures did not quite end there. After the war, he took a job in a chemical plant in Loughborough, in the English Midlands. While he was working with chlorine gas, his gas mask came loose, and he was instantly exposed to dangerously high levels of the gas. He lay unconscious for fifteen minutes before co-workers noticed his unconscious form and dragged him to safety. Miraculously, he survived. Some time after that, he was adjusting a pipe when it ruptured and sprayed him from head to foot with sulfuric acid. He suffered extensive burns but again survived. Shortly after he returned to work from that setback, a nine-foot-long metal pole fell on him from a height and very nearly killed him, but once again he recovered. This time, however, he decided to tempt fate no longer. He took a safer job as a furniture salesman and lived out the rest of his life without incident. He died peacefully, in bed, aged sixty-four in 1987.

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  —

  Now, I am not suggesting that surviving a fall from the sky is something that anyone can count on, but it has happened more often than you might expect. In 1972, a flight attendant named Vesna Vulović survived a fall of 33,000 feet when the Yugoslav Airlines DC-9 on which she was flying broke up in midair over Czechoslovakia. And in 2007, an Ecuadorean-born window cleaner in Manhattan, Alcides Moreno, fell 472 feet when scaffolding he was standing on collapsed. His brother, working alongside, was killed instantly on impact, but Moreno miraculously survived. The human body, in short, can be a wonderfully resilient thing.

  Indeed there is seemingly no challenge to human endurance that hasn’t been overcome. Consider the case of little Erika Nordby, a toddler in Edmonton, Alberta, who woke up one night in the dead of winter and, wearing only diapers and a light top, walked out of her house through a back door that hadn’t closed properly. When she was found, hours later, her heart had been stopped for at least two hours, but she was carefully warmed up at a local hospital and miraculously restored to life. She made a full recovery and became known, not surprisingly, as the Miracle Baby. Remarkably, just a couple of weeks later, a two-year-old boy on a farm in Wisconsin did almost exactly the same thing and was successfully revived and made a full recovery. Dying is, to coin a phrase, the last thing your body wants to do.

  Children do much better with extreme cold than with extreme heat. Because their sweat glands aren’t fully developed, they don’t sweat freely as adults do. That is in large part why so many of them die so swiftly when left in cars in warm weather. In a sealed car with the temperature outside in the 80s, the inside can reach 130, and no child can cope with that for long. Between 1998 and August 2018, almost eight hundred children in the United States died when left unattended in hot cars. Half were under two years of age. Remarkably—indeed, I would say shockingly—more U.S. states have laws making it illegal to leave an animal unattended in a car than to leave a child unattended. The margin of difference is twenty-nine to twenty-one.

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  —

  Because of our frailties, much of our own planet is off-limits to us. Earth may feel like a generally benign and kindly place, but a very large part of it is too cold or hot or arid or lofty for us to live successfully on it. Even with the advantage of clothing, shelter, and boundless ingenuity, humans can manage to live on only about 12 percent of Earth’s land area and just 4 percent of the total surface area if you include the seas. It is a sobering thought that 96 percent of our planet is off-limits to us.

  The thinness of the atmosphere puts a limit on how high we can live. The highest permanent settlements in the world are in the Andes in northern Chile on Mount Aucanquilcha, where miners live at 17,500 feet, but that appears to be absolutely at the limits of human tolerance. The miners themselves choose to trudge an additional 1,500 feet up the slopes to their workplace each day rather than sleep at 19,000 feet. For purposes of comparison, Mount Everest is about 29,000 feet.

  At very high altitudes, any exertion becomes difficult and exhausting. Around 40 percent of people experience altitude sickness above thirteen thousand feet, and it is impossible to predict who the victims will be because it is not related to fitness. At extreme heights everyone struggles. Frances Ashcroft in Life at the Extremes notes how Tenzing Norgay and Raymond Lambert, on a climb of the South Col of Mount Everest in 1952, took five and a half hours to climb just 650 feet.

  At sea level, about 40 percent of your blood volume is occupied by red blood cells, but that can increase by about half as much again with acclimatization to higher altitudes, though there is a price to be paid. The increase in red cells makes the blood thicker and more sluggish and puts extra pressure on the heart when pumping, and that can apply even to those who have lived their whole lives at great heights. Residents of lofty cities, such as La Paz in Bolivia (11,500 feet), sometimes suffer an illness called Monge’s disease, which produces blue lips and clubbed fingers because their perpetually thickened blood is not flowing well. The problem goes away if they move to a lower altitude. Many sufferers are thus permanently exiled to the valleys, far from friends and families.

  For reasons of economy, airlines normally keep cabins pressurized to an altitude equivalent of forty-nine hundred feet to seventy-nine hundred feet, which is why alcohol is more likely to go to your head while flying. It also accounts for why your ears pop during descent because the pressure changes as you reduce elevation. On an airliner flying at a normal cruising altitude of thirty-five thousand feet, if the cabin suddenly depressurized, passengers and crew could become confused and incompetent in as little as eight or ten seconds. Ashcroft notes the case of a pilot who passed out because he paused to put on his eyeglasses before his oxygen mask. Fortunately, the co-pilot was not incapacitated and took control of the plane.

  One of the more infamous examples of oxygen deprivation—or hypoxia, as it is more formally known—was in October 1999 when the American professional golfer Payne Stewart, along with three business associates and two pilots, was on a chartered Learjet en route from Orlando to Dallas when the plane lost pressurization and all aboard blacked out. The plane’s last contact was at 9:27 a.m., when the pilot acknowledged clearance to climb to thirty-nine thousand feet. Six minutes later, when a controller contacted the plane again, there was no response. Instead of turning west for Texas, the jet continued on a northwesterly track, on automatic pilot, across the central United States before eventually running out of fuel and crashing in a field in South Dakota. All six aboard were killed.

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  —

  A disturbingly large amount of what we know about human survival abilities comes from experiments carried out on military prisoners, concentration camp inmates, and civilians during World War II. In Nazi Germany, healthy prisoners were subjected to amputations or experimental limb transplants and bone grafts in the hopes of finding better treatments for German casualties. Russian prisoners of war were plunged into ice water to determine how long a German pilot could survive a downing at sea. Others were kept outdoors naked in freezing weather for up to fourteen hours for similar purposes. Some experiments seem to have been driven by nothing more than morbid curiosity. In one, the subjects’ eyes were injected with dyes to see if their eye color could be permanently changed. Many others were subjected to poisons and nerve gases of all types or infected with malaria, yellow fever, typhus, and smallpox. “Contrary to postwar apologies,” write George J. Annas and Michael A. Grodin in The Nazi Doctors and the Nuremberg Code, “doctors were never forced to perform such experiments.” They volunteered.*

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  —

  Horrifying as the German experiments were, they were outdone, in scale if not cruelty, by the Japanese. Under a doctor named Shiro Ishii, the Japanese built an enormous complex of more than 150 buildings spread over almost 1,500 acres at Harbin in Manchuria with the avowed purpose of determining human physiological limitations through any means necessary. The facility was known as Unit 731.

  In a typical experiment, Chinese prisoners were tied to stakes at staggered distances from a shrapnel bomb. The bomb was detonated and scientists then walked among them, carefully noting the nature and extent of the prisoners’ injuries and how long it took them to die. Other prisoners were shot with flamethrowers for similar purposes, or starved, frozen, or poisoned. Some, for unfathomable reasons, were dissected while still conscious. Most of the victims were captured Chinese soldiers, but Unit 731 also experimented on selected Allied prisoners of war to make sure that toxins and nerve agents had the same effects on Westerners as on Asians. When pregnant women or young children were needed for experiments, they were randomly snatched from the streets of Harbin. Nobody knows how many people died in Unit 731, but one estimate has put the number as high as 250,000.

  The outcome of all this was that Japan and Germany finished the war well ahead of the rest of the world in understanding microbiology, nutrition, frostbite, weapons injuries, and above all the effects of nerve gases, toxins, and infectious diseases. Although many Germans were captured and tried for these war crimes, the Japanese almost entirely escaped punishment. Most were granted immunity from prosecution in return for sharing what they had learned with their American captors. Shiro Ishii, the physician who had conceived and run Unit 731, was extensively debriefed and then allowed to return to civilian life.

  The existence of Unit 731 was a well-guarded secret, by Japanese and American officials alike, and would have remained unknown to the wider world forever except that in 1984 a student from Keio University in Tokyo came across a box of incriminating documents in a secondhand bookshop and brought them to the attention of others. By this time, it was far too late to bring to justice Shiro Ishii. He had died in 1959, peacefully in his sleep, at the age of sixty-seven after nearly a decade and a half of untroubled postwar life.

  * The insensitivity in Nazi Germany could be breathtaking. In 1941, a psychiatric hospital at Hadamar, near Limburg, celebrated the putting to death of its ten thousandth cognitively deficient person with an official celebration with speeches and beer for the staff.

  12 THE IMMUNE SYSTEM

  The immune system is the most interesting organ in the body.

  —MICHAEL KINCH

  I

  THE IMMUNE SYSTEM is big and kind of messy and all over the place. It includes a lot of things that we don’t usually think of in the context of immunity, like earwax, skin, and tears. Any invader that gets past these outer defenses—and comparatively few do—will quickly run into swarms of “proper” immune cells, which come pouring out of lymph nodes, bone marrow, the spleen, the thymus, and other corners of the body. There is a lot of chemistry involved. If you want to understand the immune system, you need to understand antibodies, lymphocytes, cytokines, chemokines, histamine, neutrophils, B cells, T cells, NK cells, macrophages, phagocytes, granulocytes, basophils, interferons, prostaglandins, pluripotent hematopoietic stem cells, and a great deal more—and I mean a great deal more.

  Some of these overlap and some do multiple jobs. Interleukin-1, for instance, not only attacks pathogens but also plays a role in sleep, which may go some way to explaining why we are so often drowsy when unwell. By one calculation, we have some three hundred different types of immune cells at work within us, but Daniel Davis, professor of immunology at the University of Manchester in England, thinks the number is essentially incalculable. “A dendritic cell in the skin will be quite different from one in a lymph node, say, and so it all gets quite muddled to define specific types,” he says.

  On top of all that, every person’s immune system is unique, making immune systems harder to generalize, harder to understand, and harder to treat when they go wrong. Moreover, the immune system doesn’t just deal with germs. It has to respond to toxins, drugs, cancers, foreign objects, and even your own state of mind. If you are stressed or exhausted, you are much more likely to suffer an infection, for instance. Because protecting us from invasion is such a limitless challenge, the immune system sometimes makes mistakes and launches an attack on innocent cells. Given the number of inspections immune cells make day after day, the error rate is really low. It is a great irony nonetheless that a very high proportion of the suffering we do is inflicted on us by our own defenses in the form of autoimmune diseases like multiple sclerosis, lupus, rheumatoid arthritis, Crohn’s disease, and many unappealing others. Altogether about 5 percent of us suffer from some form of autoimmune disease—a very high proportion for such an uncomfortable range of afflictions—and the numbers are growing faster than our abilities to treat them effectively.

  “You could look at it and conclude that it’s crazy that the immune system attacks itself,” says Davis. “Alternatively, once you start to think about all that the immune system has to do, it’s surprising that it doesn’t happen all the time. Your immune system is constantly bombarded by things it has never seen before, things that may have only just come into existence—like new flu viruses, which are constantly mutating into new forms. So your immune system has to be able to identify and fight off a more or less infinite number of things.”

  Davis is a big but gentle man in his forties with a booming laugh and the happy air of someone who has found his niche in life. He studied physics at Manchester and Strathclyde Universities in Britain, but then moved to Harvard in the mid-1990s and decided that biology was where his real interests lay. By chance he ended up in the immunology lab at Harvard and became gripped by the elegant complexity of the immune system and the challenge of trying to unravel it all.

  Despite the intricacy at the molecular level, all parts of the immune system contribute to a single task: to identify anything that is in the body that shouldn’t be there and, if necessary, kill it. But the process is far from straightforward. A lot of things inside you are harmless or even beneficial, and it would be foolhardy or a waste of energy and resources to kill them. So the immune system has to be a bit like security people at airports watching stuff on a conveyor belt and only challenging those things that have nefarious intent.

  At the heart of the system are five types of white blood cells: lymphocytes, monocytes, basophils, neutrophils, and eosinophils. They are all important, but lymphocytes are the ones that excite immunologists most. David Bainbridge calls lymphocytes “just about the cleverest little cells in the whole body” because of their ability to recognize almost any kind of unwanted invader and mobilize a swift and targeted response.

  Lymphocytes are of two principal types: B cells and T cells. The B in B cells comes, a little oddly, from “bursa of Fabricius,” an appendix-like organ in birds where B cells were first seen.*1 Humans and other mammals don’t have a bursa of Fabricius. Our B cells are made in the bone marrow, but it is entirely coincidental that that starts with a b, too. T cells are more faithful to their source. Though created in the bone marrow, they emerge from the thymus, a small organ in the chest just above the heart and between the lungs. For a very long time, the role of the thymus in the body was a complete mystery because it seemed to be just full of dead immune cells—“the place where cells went to die,” as Davis put it in his superlative book The Compatibility Gene. In 1961, Jacques Miller, a young Franco Australian research scientist working in London, unraveled a mystery. What Miller established was that the thymus is a nursery for T cells. T cells are a kind of elite corps in the immune system, and the dead cells found in the thymus were lymphocytes that had failed to pass muster because they were either not very good at identifying and attacking foreign invaders or because they were too eager to attack the body’s own healthy cells. They had, in short, failed to make the cut. It was an immensely significant discovery. As the medical journal The Lancet observed, it made Miller “the last person to identify the function of a human organ.” Many people have wondered why he has never been honored with a Nobel Prize.