LITTLE MORE THAN one hundred years ago, medicine meant crude and dangerous practices: draining blood from the body in an effort to rid a person of disease, amputation, doses of addictive morphine to kill pain. These methods were often ineffective at best and deadly at worst. Medicine was powerless to stop many from dying of childhood diseases, such as scarlet fever and whooping cough. The influenza, or flu, epidemic of 1918 killed 675,000 Americans and more than 21 million people worldwide in just a few months.

Today, vaccines have all but wiped out many once-common diseases. Medicines effectively treat people who fall ill from a wide variety of infections and other illnesses—and who would have died from those illnesses fifty or one hundred years ago. Doctors today know how to perform lifesaving surgery that was unheard of in the not-so-distant past. Progress in medical science...