[tw25508394]

Single-cell organisms replicate.

Single cells of multi-cellular organisms replicate.

Shoot me in the head and all my cells will eventually stop replicating.

To a cell in my body, is the body a host or an environment? Is the distinction useful?

[est31]

I would say neither. Your body is a colony, like an ant colony or bee colony. In all instances, most individuals can't start a colony of their own but it's limited to a set of queens/gametic cells.

As for shooting into the head, some multicellular life forms can actually heal from that. From planaria you can remove any part of their bodies, it will grow back. Shooting into an ant colony won't do much harm to them either, nor is removing the branch of a tree. It's only a property of animals I'd say who have given up this property (for the most part) in exchange for more complex metabolic systems. (heart, nervous system, etc)