Our great enemies are the viruses. These pesky little parasites have no mitochondria - they are unable to produce their own power for living. From time to time, our membrane guardians fall down on the job and a virus penetrates a cell. With power now available, these terrors start reproducing. Overwhelmed by virus particles, the unfortunate cell perishes. Then the released virus attacks other cells. In even the mildest virus infections, millions of cells perish. If it were not for a variety of body defences, the viruses would take over and you would not be long for this world.