When the Great War annihilated civilization, the survivors lost their loved ones, their homes, and their worldly goods. They endured a daily search for food, water and medicine, but they also had to seek out their lost identities. People have always defined themselves by the work they do, the place they live, the things they own. In the Great War it wasn't just buildings that were destroyed - entire cultures were erased from history. In the brutal years that followed, the survivors had to define themselves again, find meaning in their existence, or face a metaphorical loss of their humanity. But a much more literal threat to their humanity was already in place, waiting quietly beneath the desert for someone to stumble across it. Someone did, nearly a century after the war, and that threat came closer to extinguishing humanity than all the bombs and guns of the old wars.
A single neutron passing through a strand of DNA can cause a cancerous tumor, but a lifetime of exposure to unprecedented amounts of atomic fallout will have wildly unpredictable results. After a few generations of passing on this defective DNA a species will begin to mutate. That explains many of the strange beasts running around the wasteland. But not all of them.
There's a different strain of mutant out there. They're shaped like men, but... bigger. Some of them are more dangerous than deathclaws, stronger than a man in power armor. They call themselves "Super Mutants" and they're not the result of random contact with radiation, but rather a deliberate act of genetic tampering.
In the years leading up to the Great War the most powerful nations became desperate for ways to outmatch their foes. Deadlier guns and ruthless propaganda created armies of fanatical solders. Still, no matter how well conditioned or well armed a soldier is, they are still only human. But that can change.
Holotapes unearthed in pre-war military installations show that the United States government started out with good intentions. Rival nations had attacked America with biological weapons, resulting in a "New Plague" so America's leaders created their own defensive virus. The plan - originally - was to make a germ that would mutate America's population just enough that they would become immune to the viruses that the enemy might use.
It was an idea so outlandishly daring that it seemed worth the risk to a government that faced imminent destruction. It could force the population to instantly evolve to a point where they were naturally immune to the New Plague - whether they wanted to or not.
In a sense this Forced Evolutionary Virus worked, but the effects were much more drastic than anticipated. The animal test subjects mutated by it were not only resistant to common biological weapons, they also grew stronger and more intelligent, but monstrously deformed.
In those last days of patriotic fervor there would have been plenty of soldiers willing to become human guinea pigs to test the Forced Evolutionary Virus or FEV. Lucky for them the war ended before the government started asking for volunteers. Huge vats of the virus were secured away in facilities on both sides of the country.
In Maryland the virus was placed in a Vault-tec Vault, and used as part of their cruel experiments. According to files recovered from a Vault-tec mainframe, the people who took shelter in that vault were exposed to the virus by their own Overseer, the very man assigned to protect them after the bombs fell. After escaping the war, these people suffered even worse than those trapped on the surface.
The mutation process is painful, and most people exposed to the virus die a horrible death in a body they don't even recognize as their own. Those who do survive often wish they hadn't, because their very humanity is taken from them. Whatever they looked like before mutating, they transform into an enormous, bald, green brute. Back then lots of people defined themselves by skin color. After mutating they were all just varying shades of green. Women lost any shred of their original gender and became indistinguishable from the male mutants. And the men? Some of them even lost the very thing that made them men.
