I agree that it should somehow be killable, but I really like the concept.
Personally, I would say that the mothers should be normal monsters. The potential for putting them smack in the middle of the room is akin to the following formation of mud (m is mud, M is mud mother):
WW..WW
WmmmmW
.mMMm.
.mmmm.
WmmmmW
WW..WW
The architect can make them unkillable.
But here's a twist: how about they are highly volatile and as a result an explosion will follow the Virus and burst up to one square beyond the normal radius, including onto the ground and not limited by the path of the explosion. That way you can blow holes into the walls with bombs. And the ability to carry bombs around corners is pure puzzle potential, folks.