How Much Of You Is Alive?

Bits of you are always breaking down, being thrown out, and being replaced. Even right now, parts of you are dying. Some of your cells even died before you were born. And some will never come back. These are all very strange, mildly uncomfortable things to think about. And what's even stranger than that is a big fraction of your body is, was, and will never be alive. So how much of you is dead?