John Maler

“Even at rest we sweat steadily, if inconspicuously, but if you add in vigorous activity and challenging conditions, we drain off our water supplies very quickly. According to Peter Stark in Last Breath: Cautionary Tales from the Limits of Human Endurance, a man who weighs 155 pounds will contain a little over forty-two quarts of water. If he does nothing at all but sit and breathe, he will lose about one and a half quarts of water per day through a combination of sweat, respiration, and urination. But if he exerts himself, that rate of loss can shoot up to one and a half quarts per hour. That can quickly become dangerous. In grueling conditions—walking under a hot sun, say—you can easily sweat away ten and a half to twelve and a half quarts of water in a day. No wonder we need to keep hydrated when the weather is hot. Unless the loss is halted or replenished, the victim will begin to suffer headaches and lethargy after losing just three to five quarts of fluid. After six or seven quarts of unrestored loss, mental impairment starts to become likely. (That is when dehydrated hikers leave a trail and wander into the wilderness.) If the loss gets much above ten and a half quarts for a 155-pound man, the victim will go into shock and die. During World War II, scientists studied how long soldiers could walk in a desert without water (assuming they were adequately hydrated at the outset) and concluded that they could go forty-five miles in 80-degree heat, fifteen miles in 100-degree heat, and just seven miles in 120-degree heat.”
― Bill Bryson, The Body: A Guide for Occupants

“Another huge boost can be attributed to vaccines. In 1921, America had about 200,000 cases of diphtheria; by the early 1980s, with vaccination, that had fallen to just 3. In roughly the same period, whooping cough and measles infections fell from about 1.1 million cases a year to just 1,500. Before vaccines, 20,000 Americans a year got polio. By the 1980s, that had dropped to 7 a year. According to the British Nobel laureate Max Perutz, vaccinations might have saved more lives in the twentieth century even than antibiotics. The one thing no one doubted was that practically all the credit for the great advances lay securely with medical science.”
― Bill Bryson, The Body: A Guide for Occupants