Parasites imply a host.
Tyranids don't use hosts. They just eat the stuff out of whatever they find and make more Tyranids.
Same for Zerg.
Yeah man just possess an extra-dimensional eldritch abomination. Sure. Why not. At the same time I think I'm gonna go take this Tyranid swarm and have them devour some Necrons, see if I get some loving metal Tyranids.
Why the forget not.
Elaborate because you sound like an idiot right now
When a Zerg Queen captures a Terran and converts them, parasitic properties.
Nids, well they kinda steal genes for their own benifit.
and sure, why not.
If C'thulu can exist in a physi-oh wait. Nevermind, people, C'thulu is out of the question now.