General overview  
The Zerg actually encompase a multitude of bio-engineered creatures, designed for maximum killing efficiency. Their hives are also living creatures which slowly infest any area the Zerg inhabit with "the Creep", a thick blanket of organic matter.
The Zerg (origin unknown) are actually composed of several different types of creatures ("breeds") which, while physically distinct and unrelated, seem to share a common psychology and sense of community. Although we have had only limited resources to draw upon--mostly sensor logs, eyewitness accounts, and expired subjects--the picture that we have created is a fascinating one. Perhaps the one thing that we do know is that the Zerg do not wish to coexist with any other species. They are extremely aggressive and almost cruel in their attacks and attempts to infest inhabited areas. Species that they encounter are either deemed useful to the hive and mutated, or eradicated.  
 		 Physiology  
The Zerg as a whole are extremely tough, tenacious, and deadly. Their natural armor and weaponry is comparable to modern technology in terms of toughness and armor-piercing capabilities. They are extremely resistant to radiation and chemical agents, and possess an incredible regenerative ability and accelerated 
growth rate. Great care must be taken to 
completely destroy any Zerg encountered, or they 
will return in full health.  
  
  
  
  
 
 		 Psychology  
I would be hard pressed to determine the full extent of the Zerg intelligence, but it is clear that they are a very cunning species, and they are capable of higher reasoning. Although they do not use technology that we know of, they demonstrate a clear understanding of it, and have been known to open airlocks and doorways in colonies that they have raided. As a group they also react intelligently to attacks and are adept at tactical combat. On an individual basis, though, the Zerg seem to be hardly more than savage animals with little or no concern for self-preservation.