Hosts do the replication for the viruses.
Environments do NOT do the replication for the fish.
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?
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)
Mammals are the exception here, there the environment for replication is an integral part of the body of the parent. But in all those other cases, including, technically, marsupials it isn't.
An egg is a complex life that can reproduce the whole organism (and actually with energy stored INSIDE, that’s why they are great nutritios value for others)
