Orks as a race are extremely tough and resilient; the only sure way to definitively kill one is decapitation. Many an unfortunate opponent has had the nasty surprise of seeing the Ork they had just disembowled keep fighting barely hampered by the terrible wound. The Greenskins' resilience and regenerative capacities have their limits however, hence the "Brain Boyz" (as their creators are known to the Orks) saw fit to provide the Orks with medical specialists with a genetically encoded knowledge of and passion for the inner workings of the Orkoid body.