Imagine a cell nestled happily in the human body and enjoying the best of all possible worlds. It is endowed with immortality, the remarkable ability to divide indefinitely. Each time it cleaves, it makes two daughter cells with different fates. One divides again and again and again, spawning hundreds of copies of itself before exhausting its powers of duplication and dying out. The other progeny is a bit more cunning, inheriting from its parent the gift of never-ending life. That cell resists the temptation to multiply and march to an inevitable death, choosing instead to divide only occasionally and, by...