Some of our cells stop replicating and continue to live in a way that is harmful to our bodies. These are cells that enter a state of “replicative senescence”, i.e. senescent cells. They stop replicating and don’t die. These cells become immortal, thus called “zombie cells”. They won’t die naturally and as long as nothing is done to clear them, they will live as long as you will and they will accumulate to a point where they become extremely harmful to you.