"The Daemon lied with every breath. It could not help itself but to deceive and dismay, to riddle and ruin. The more we conversed, the closer I drew to one singularly ineluctable fact: I would gain no wisdom here."
Gender: Male Location: 1/9.7'rd Horseman of the Apocalypse
They seem to largely lack that, though, it's more the domain of the Protoss. The Zerg's only psychic nastiness, unless I'm tired and retarded, seems to be Kerrigan. They have biological nastiness like Defilers and Queens, but I don't think these things will allow them to kill Metroids or X Parasites.
The X are parasitic life-form native to SR388. They reproduce asexually and rapidly in their natural environment, going from perhaps four or five individual parasites to hundreds, at least. They seek out hosts to inhabit and absorb their forms and knowledge from within. The Metroids are the main predators and natural enemies of the X, capable of absorbing the parasites life-force en masse to keep the species in check. In Metroid Fusion, an X infection almost kills Samus Aran, and drastically alters her physical form. A vaccine developed from the cell culture of an infant metroid, the baby Metroid, in fact, saves her life, killing off the infection. The few specimens left attached to her old Power Suit were shipped to a research station orbiting SR388, and the X were able to manifest a copy of Samus with the full complement of her suit functions, and after breaking free, the X proceeded to overrun the station. Only the timely arrival and intervention of Samus prevented the X from spreading across the galaxy. I'd add more but at this point I'm dipping into the main plot of Metroid Fusion.
I think what makes them so deadly is their tendency to absorb theirs hosts completely, essentially taking their form and all the knowledge they pertain. If the X were to, say, absorb a Cerebrate or something, they could wreak havoc amongst the Zerg, not to mention all the absorbed "Xerg" that would come into being. I'm not sure what they would do about creep, but again, these are just my basic opinion and reasoning as to why I think the Zerg would have trouble fighting the X.
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What the X do is take DNA, and make a copy of what they infected. Then that copy asexually divides, and those copies asexually divide, and so on and so on. Their ability to kill you via shutting down the nervous system and/or just eating you is not the real threat, so much as the fact that they'll copy all your abilities and then proceed to paste them hundreds of times.
No, I suppose I just understood "natural enemies" wrong.
Well, the Zerg would be physically taking over whatever in the same fashion that the X-Parasite does, it appears. If there aren't anyones there to take over that can fight back the X-Parasite, though, I suppose they have to rely on their natural resistances and regenative abilities, as well as sheer numbers.
I assume it's visible whom the X-Parasite take over? From how I understood it, the parasite deform the physical form. So unless the X-Parasite outnumber the Zerg, the Zerg could just as well kill those of themselves that get infected.
Killing the host does not eliminate the X Parasite. In fact, when you do that the X Parasite just flies off, and if you wait long enough one parasite will come back and reform it. The only reason Samus can even fight them is her Metroid vaccine allows her to absorb them without being killed. It's how she heals in that game.
All hail Scythe, King of the Sigs.