Forced Evolutionary Virus (Psyker Canon)

The Forced Evolutionary Virus, or FEV, is an artificial virus originally created by West Tek's NBC Division. Its ability to force changes in both the genotype and phenotype made it one of the most important mutagens in the wasteland, harnessed by different organizations and individuals for their own purposes both before and after the Great War.

Unknown to the rest of the world, multiple laboratories managed to acquire samples of FEV and modified it for their own research and testing. One strain, considered the ultimate success by the Institute, is the 'Izanami' strain of the FEV.

Early History
During the peak of scientific development prior to the Great War, many nations, if not all, tried to make better weapons along with scientific and technological advancement. Among them being in bioscience, which ultimately led to the research of the Forced Evolutionary Virus, which multiple countries attempted to weaponise and improve.

Despite the genotoxic nature of the substance, the nations that were aware of it's existence continued their research on the FEV, with the Japanese working with a strain dubbed as Izanami, named after the Japanese goddess of the underworld due to it's highly toxic and mutagenic nature. Izanami was known to be of low virulence and required direct transfusion of blood to infect new hosts, but it was known to kill rather than mutate those it was used on.

Because of the toxic nature of the Izanami strain, it was only injected into hospital patients used as guinea pigs without the knowledge, including cancer patients, causing mutagenesis among those who carried defective DNA rather than healthy cells. The surviving subjects became the first 'Psykers' with weak abilities, their cancer cells reverting to relatively healthier cells and replacing most of their dying body cells.

This breakthrough made the scientists aware of the project continue into further research on it, widening their search until patients who kept their fertility or had it restored post Izanami infection were found. These subjects were able to pass on their genetic mutations and their children exhibited both stronger abilities and better health. Izanami FEV was later used on said children and the survivors again chosen to see if even stronger psykers were able to be made.

After a few generations of refinement, the first 'perfect Psykers' were born with no health issues and strong abilities, among these being an individual named Kyouya Kasuga. Kyouya was involved in clandestine missions after he grew to adulthood, and it was known he was sent to the United States before the bombs fell.

Findings of the Institute
When the Institute recovered the son of Kyouya Kasuga, Shaun, from Vault 111 after recovering records of the underground facility, they found that Izanami FEV was bound in the infant's DNA within the then unknown Izanagi Regions. The Izanagi region was a mutant human region within the chromosomes that allowed proper insertion of Izanami and subsequent genetic rewriting. It served as a template for self replication and insertion of the Izanagi region and Izanami FEV insert into homologous chromosomes. Thus the origins of Psyker powers was determined.

It was later realized by the Institute after further studies that the Izanami FEV was not actually a potent mutagen and toxin. Rather, Izanami FEV forcefully purified a human being into a higher life form, explaining the deaths of normal, healthy people exposed to the strain since some damaged genes had a higher homology to the Izanagi regions. This allowed Izanami FEV's proper insertion and rewriting of the genome. This created the paradox that people with genetic errors and cancers managed to survive Izanami while the healthy could not. Most of their cells were destroyed, but their cancer could be directed by Izanami to repair the body, and so forth.

The Institute, having studied FEV but not aware of the Izanagi regions when they first obtained Shaun, purified the FEV from Shaun's cell extracts. They ended up cloning the Izanagi Regions into their Gen 3 Synths when they tried to fill the gaps of their Synth research with Shaun's DNA.

Eventually, when Synths escaped through various reasons and methods (e.g. the Railroad,) some came into contact with FEV (such as those from the Commonwealth's Super Mutants) causing the reactivation of their Izanagi Regions, which caused genomic rewriting and power awakening. This transformation also allowed their bodies to break down and absorb their Synth components.

Trivia

 * Izanami FEV causes production of uncharacterized proteins and RNA species, which allow a Psyker to extend a "psychic" field from themselves; This field can directly transform reality to their effect.
 * Synths which have transformed into psykers are visually indistinguishable for normal humans. They are able to grow and reproduce, unlike normal synths.