Nearly 50,000 people in the United States die each year because there are not enough kidneys for transplant, which adds up to more than double the number of annual murder victims. Hundreds of thousands more are on dialysis, a lifesaving but time-sucking and physically draining treatment. Humans need only one kidney to live, but we have two. Giving away my kidney, to a 23-year-old woman I didn’t know, has been the most fulfilling experience of my life.