Earlier this year, I watched someone die from evolution. I was spending a few days working with infection specialists at a major U.S. teaching hospital. Infection specialists get called in when patients get infections that their attending physicians can not cure. One of the first patients I saw had a bacterial infection in his blood stream. He had severe breathing difficulties and was unconscious. I was struck by how colorless he was. A nurse was putting him on a ventilator. When I saw him the next day, he was still alive, but the ventilator was working so hard, he did not have long to go. The day after, I watched his family decide to turn the ventilator off. He had gone. The bug that killed that man most likely got into him a week or so earlier during a minor surgical procedure in that same hospital. So, in one of the most advanced hospitals in the richest country in the world, a man got infected and died. The incredible thing is that he could have been easily saved when I was a college student. Then, the bug that killed him was easily treated with antibiotics. A quarter century later, those drugs do not work well. That is because the bugs have changed. They have evolved. It is hard to know exactly how many people die of infections that were once easily treated. In Europe, it might be 25,000 a year. In U.S., the figure is somewhere north of about 40,000. It might even be 100,000. Even 40,000 a year, that is more Americans than are killed each year in car crashes. You all know somebody who's died in a car crash. If you don't already know someone who's been killed by an infection, you soon will. Cars have been around longer than these drug-resistant bugs. Before modern medicine, death from bacterial infections was common. The first person to be given penicillin, got his infection from a scratch on a rose bush. He died from that infection because at that time, in the 1940s, the entire world supply of penicillin went into him, but it was not enough to save him. Since then, production processes have become much more efficient, and a wide range of antibiotics have been developed. And they really are wonder drugs, they're magic bullets. For a few dollars, a few pills can save your life. The second half of the 20th century was a wonderful time to be in medicine. In the 1960s, people started to say the war on infectious disease was won. But then evolution happened. Evolution is the change in inherited characteristics of a population over time. Many people think evolution is about dinosaurs and whether we are descended from monkeys. And it is. But the same process that made dinosaurs and us is what killed the man on the ventilator. His bug was descended from a long line of bugs that had been infecting previous patients, each of which was treated with antibiotics. Some of those bugs had mutations which made them less likely to be killed by the drug. Each time the drug worked, ancestral bugs were killed, and the mutants were able to take over. And so the evolutionary process ran, ultimately leading to the untreatable infection that killed that patient. Modern medicine drove that evolutionary process. The decisions about how to use drugs, that were made by patients and doctors upstream in the transmission chain, leading to that man, ultimately killed him. The decisions being made by patients and doctors today, are determining your chances of dying of an infection in the future. Evolution is everywhere in medicine. We essentially wage chemical warfare on the life forms that harm us. In response, they evolve resistance. This is true when you use insecticides to attack mosquitoes, which transmit diseases like Dengue or Malaria. Insecticide resistant mosquitoes evolve. It is true when we use drugs to attack the viruses, bacteria, and even the worms that make us sick. Drug resistance almost always evolves. And it is especially true, when we attack cancer with potent anti-cancer drugs. In rich countries, no one dies of drug-sensitive cancer cells. If you or your family or friends die from a tumor, it will be filled with cells that have evolved resistance to chemotherapy. Every year, over 500,000 Americans die of drug-resistant tumors. Add to that, those dying of drug resistant infections, and that means that more than 600,000 Americans die each year from evolution. Think about that. 600,000 Americans die each year from evolution. It is as simple as that. Evolution kills a lot of people. Sometime, as evolutionary processes are slow, dinosaurs dominated the earth for more than a hundred million years, during which time dinosaurs of many sizes and shapes evolved and went extinct. But disease-causing agents can evolve to cause problems over very short time scales. This is because they often have very short generation times. Weeks, days, or even hours, and there are often billions of them. This generates very potent evolution. Just imagine, your intestinal tract contains more bacteria than there are cells in your body. You use an antibiotic that kills all those bugs except a few resistant ones. Billions of competitors are now gone. Those resistant bugs can replicate like crazy. Two, then four, then eight, then 16, 32, and 64, and in no time your gut has billions of resistant bugs. Those resistant bugs have taken over their world, and the wonder drug no longer works. Evolution is a controversial topic, especially in America. That means it is often poorly taught at schools and in universities. It is possible to become a physician in America, not believing that evolution happens, or not knowing much about how it works. I wonder how many people would be alive now if evolutionary science was thoroughly taught to everyone? More and more people will die of drug-resistant infections as those bugs spread. More and more of us will die of cancer as we live longer. Evolution matters in medicine. It should matter to you, many of you will die from it.