We marvel at modern medicine's ability to heal life-threatening injuries, prolong lives, and cure diseases. And yet, as any physician will tell you, medicine is an imprecise science. A cell undergoes permutation or an organ fails, and no number of pills or procedures can help. A full understanding of the intricacies of the human body—the fragile sack of liquids, organs, nerves, and bones that propels us through our brief, mortal existence—remains elusive.