Meet the Virus. It doesn't eat. It does not breed. It does not use any chemical reactions. It cannot change or move. They are more related to you than your cousins.
Despite this, it is one of the biggest killers in the world. Other cells are tricked by their outer coating, and absorb the virus into itself. Then the virus DNA is forced into the cell DNA, forcing the cell to create more copies of the virus. The cell will do this until either it has nothing else to use or there are too many virii in the cell. Then it bursts open, releasing the virii.
This virus in particular is a rather nasty one. It has an outer coating of sugar as to trick the immune system. It then attacks these immune cells, killing them gradually. Normally the immune cells would learn from this and produce antibodies to counter this virus. However, there is a problem with antibodies. A specific type of antibody is tailored to neutralize one and only one type of virus. Thus the virus mutates every generation, making itself a completely different mutant every generation. by the time the original generation has been killed off, ten more are out there.
Now this virus does not kill. However, it kills off the immune system of it's host. Thus other nasty things like viruses, bacteria and fungi can enter the body and cause damage. This virus has singlehadedly brought back diseases once thought eradicated.
This virus is the HIV virus.
It is not technically alive.
It will kill you before ten years have passed unless you have good doctors, doctors that even only the rich can afford.
It will kill you painfully. Your flesh will be eaten away, and fungi will cover your skin with their colonies. Your brain will start to rot, and toxins will be constantly pumping through your body.
It cannot be detected until, at the very least, three weeks have passed.
It is contagious from day one.
HIV is the zombie plague of the virus world.