Raiders

Before the Fall, there was one humanity. After it, there were two.
  The Lineages chose symbiosis. They accepted the Infection into their bodies, gained its gifts, paid its prices, and built a civilization on the bones of the old world. They called themselves Homo symbioticus, though most just call themselves survivors. The other branch refused. Whether by biology, by circumstance, or by sheer violent refusal, the Raiders rejected the Infection entirely. Evolution did not care about their reasons. It simply pushed them down a different path: faster, stronger, harder, and wired at the genetic level to destroy anything carrying the symbiotic payload that the Lineages depend on to survive.
  The scientific name is Homo insurrectus. The wasteland just calls them Raiders.
  Three subspecies have emerged from this divergent branch, each shaped by different pressures. The Aberrant Raiders (Homo insurrectus aberrans) survived partial Infection exposure and came out the other side twisted: mutated, feral, and shunned by their own kind. The Infected Raiders (Homo insurrectus morbidus) were once Lineages, transformed by Bad Brain disease into something caught between what they were and what they have become. The Natural Born Raiders (Homo insurrectus ferox) are the purest expression of the species: uninfected, unaltered humans who have built warrior societies capable of fielding armies that number in the tens of thousands.
  What binds all three is Raider Rage. When a Raider activates this biological response, their body floods with a combat state that restores all physical damage, refreshes every capability they possess, and renders them functionally invulnerable for thirty seconds. It is not adrenaline. It is not willpower. It is an evolutionary adaptation, a survival mechanism built into the species at a level deeper than conscious thought. Raiders can trigger it before taking a single wound, using it as a weapon rather than a last resort. The Lineages have nothing equivalent.
  The other binding factor is the Mortis Amaranthine, or rather, their exclusion from it. Lineages pass through the Mortis when they die, returning changed but alive. The vast majority of Raiders face permanent death. Only the Infected, carrying residual fragments of the symbiosis they lost, can force their way through the Mortis at all, and they do so violently, chaotically, leaving disease in their wake. Aberrant Raiders are biologically incompatible. Natural Born Raiders lack sufficient Infection to even begin the process. For most Raiders, every fight is the last one. That desperation shapes everything about how they approach violence.
  Negotiate with a zombie; at least it does not understand hate. A Raider does. They share our ancestors but chose a different path, and now they are hardwired to eradicate everything the Lineages have built. Understanding the three subspecies, their biology, their tactics, and their threat levels is not academic curiosity. It is survival.