FEV nears completion. Test on lab animals are at a near 100% success rate. Size and muscle density increase approximately 60%, and the potential intelligence increase by 200%. Effects upon human subjects remain unknown; although they are theoretically promising. The military, wishing to continue further testing, builds a large facility at the Mariposa military installation in central California. At this new facility, testing of the FEV virus continues on volunteer subjects from the military.”— FEV research log, recorded in 2077.
Research into possible cures for the New Plague created the Forced Evolution Virus, which was further developed in an attempt to create a transmittable genetic-engineering virus -- in effect, infectious evolution.”— ZAX 1.2
The Forced Evolutionary Virus (FEV/F.E.V.) is a major source of mutation in the Fallout universe. Super mutants, the Master, and possibly some ghouls are a result of infection by the FEV. Aside from super mutants, the Master also created a variety of other aberrant creatures using the virus, which includes, but is not limited to: floaters, centaurs, and potentially deathclaws. The FEV exists in various forms across all of the Fallout games, mainly to serve goals of the various antagonists. It was through the FEV that the Master made his super mutant army and how the Enclave meant to purify the world of mutation.
The FEV's composition is known and gone into detail solely by ZAX 1.2, however the effects are documented and/or described by ZAX, those who used it such as the Master, the Master's Lieutenant, Charles Curling, and Weston Lesko, as well as those who were mutated by it such as Harold, Talius, Marcus, Fawkes, and Tabitha.
According to ZAX 1.2, the FEV is a shifting-absorptive megavirus with a protein sheath reinforced by ionized hydrogen that makes it immune to irradiation. It is pre-programmed with specific introns of genetic information for the species of the infected/inoculated specimen. The FEV works by copying the host cell's DNA, like RNA, and placing them into exons that are then re-introduced into the host's cells alongside the pre-programmed introns. What makes the FEV unique is that it utilizes a recursive genetic code that allows it to repeat its genetic copying and insertion infinitely. 
Common amongst sources is that the FEV operates more undesirably on subjects with heightened mutation to their, otherwise, pure genomes as well as sterility in those infected. 
According to ZAX 1.2, the sterility is the result of the FEV seeing "half-cells" (haploid gametes) as genetic damage and correcting it. The sterility effect is a common cause of concern for pro-Mutant agendas promoted by the Master, Gammorin's Army, and Attis. A solution for the sterility is potentially pursued by Gammorin's Army along with Brotherhood of Steel (Midwest) General Dekker as a bargaining chip against Gammorin's Army.
Radiation is frequently blamed as the cause of this genetic tainting with the sole exception of the Master's Lieutenant who cites immunity from mutated airborne strains of the FEV as the cause instead. 
Upon expression, the FEV genes typically increase muscle and brain mass. 
Only one species remained fertile with further exposure to FEV. Deathclaws have been known to exhibit increased intelligence while still being able to reproduce, as is the case with the Vault 13 Deathclaws.
According to ZAX 1.2, the FEV is incapable of being air-transmissible which, if true, contradicts the Master's Lieutenant's immunization theory. In the decades to come, however, Charles Curling managed to create, through alteration, a strain of the FEV that is transmissible via "aerosol solution".
The FEV is typically inoculated into subjects who are brought into direct contact with a FEV sample such as at the Mariposa Military Base or injected into subjects directly. 
History of the FEV
Before the Great War
In the years immediately preceding the Great War of 2077 between China and the United States, both sides were scrambling to research a new weapon that would give them an edge in the coming conflict. Chinese government-funded programs began to research invisibility, while the United States government tapped the resources of American private industry to develop a secret biological weapon that could defeat the Chinese. One company, a high-technology scientific research firm known as West Tek, was already a major American defense contractor and quickly became involved in tipping the global scale. In the first Fallout, the Vault Dweller explored a location in New California called the Glow, which was in fact an underground West Tek research facility that had been directly targeted by Chinese nuclear warheads during the Great War. While exploring the highly radioactive ruins, the Vault Dweller discovers that West Tek had created a number of useful, yet volatile biological, chemical, and conventional weapons for the American military. Potent laser and plasma weapons were being experimented within the facility, and power armor, massive self-contained suits of mechanized combat armor that turned a single soldier into a walking tank, were also developed there. Most importantly, research into immunity-boosting drugs designed to combat biological weapons agents was being conducted by West Tek which lead to the creation of the PVP, and later, the FEV.
One fear amongst the American military of this period was that the Chinese would deploy biological weapons against American troops; these would be powerful, genetically engineered super-viruses that would decimate other armies and civilian populations without a single shot being fired. Thus, West Tek had initiated the Pan-Immunity Virion Project (PVP) to develop a way to stimulate the human immune system in such a way that it could naturally counteract any Chinese biological agents. It was determined that altering the subject's DNA in such a way that its dual helical structure transformed into a quadruple helical structure would render the recipient immune to almost any known or engineered pathogen. During West Tek's testing phase of the PVP in 2075, abnormal side effects were observed developing in the test subjects. The test animals began to grow dramatically larger in size and muscular structure, and their brain activity increased. Seeing the potential military applications of these unexpected results, the U.S. Army sent its own military scientists into the facility to secure the project and renamed the PVP virus as the Forced Evolutionary Virus (FEV). Not content with producing mere immunity to disease, the Army's researchers hoped that the FEV would turn its soldiers into abnormally strong and intelligent supermen capable of defeating many times their own number of Chinese soldiers. When the testing on animals reached its logical conclusion, the American military moved the entire apparatus of FEV research to the Mariposa Military Base in California, where secret human testing on military prison inmates proceeded.
The Great War
However, the grim nature of the research being carried out at Mariposa greatly unnerved many members of the U.S. Army garrison guarding the base. The nature of the military scientists' work, which had until then been kept a secret from the Army grunts, caused them to revolt when they discovered the truth. Colonel Robert Spindel, the officer in charge of the garrison, lost his mind and committed suicide. His second-in-command, Captain Roger Maxson, took command of the troops and began interrogating the Mariposa scientists. Rattled by the vile things the scientists were doing to their test subjects and incredulous of the claim that the science team was only following orders that had been handed down from the highest levels of the Pentagon, Captain Maxson had the researchers executed. Later, he announced the Mariposa Military Base to be in full secession from the United States, and his own unit to have left the U.S. Army. Only days after these actions, in October 2077, the Sino-American War reached its climax as nuclear bombs and warheads fell from the sky and the United States ceased to exist as a unified nation. Disgusted with the Mariposa base, and haunted by the fate of the FEV test subjects along with the executed scientists, Maxson decided to lead his troops and their families to the federal government's Lost Hills emergency bunker, where they would later use their high-tech military equipment and knowledge brought from the Mariposa base to form the Brotherhood of Steel.
Richard Grey's Expedition
The Mariposa Military Base remained silent for a long time after the bombs fell. Some time after the Great War, however, a group of explorers from New California, including the future mutant Harold, entered the base looking to end a peculiar proliferation of mutated animals which seemed to be coming from the installation.
Leading the expedition was Dr. Richard Grey, previously known as Richard Moreau until he was exiled from Vault 8, one of the massive underground bunkers created by Vault-Tec Industries as part of Project Safehouse. When Grey's team entered the Mariposa base, most were killed when they encountered the base's still active automated robotic defense systems. Eventually, only Grey and Harold are known to have remained of the original group. Harold was knocked out and somehow woke up out in the desert, with the appearance of a ghoul without actually having any of the traits of one. Grey, however, was heading for bigger trouble. He had made it to the vat room, where massive, bubbling cauldrons of the thick green nutrient solution that contained the FEV awaited to be used in experimentation. Grey was knocked into a vat by an automated crane and managed to crawl out after a month of complete exposure. While the FEV was designed to be injected into humans, Grey soon found out that direct physical contact with the nutrient solution that it grew in worked just as well. He began to rapidly mutate in horrible ways, turning into a tentacled blob of mutated flesh, but he also developed potent psychic powers, which were enhanced by consuming other living minds to expand his own psionic strength. As other wasteland wanderers explored the base over the years, Moreau eventually began experiments on these human subjects using primitive physical contact with the FEV.
The Coming of the Master
Grey found that many of his test subjects were turning into giant brutes with the intelligence of a child, if they survived the mutational process at all. However, a rare few test subjects became the meta-humans known colloquially as super mutants. Immune to both disease and ionizing radiation and blessed with exceptional strength, intelligence, and endurance, super mutants were superior to pure-strain humans in every way. They also possessed the exceptionally long life common to those individuals infected with FEV, as well as (unbeknown to Grey at the time) total sterility. Grey - who now called himself the Master - had long lamented the needless destruction of human civilization caused by the final nuclear holocaust of the Great War. In his warped state, he decided that he would have to force humanity to evolve beyond its own destructive impulses. If everyone could be as perfect as his super mutants, there would no longer be any reason for human conflict and the misery it wrought. Those who could not evolve along the lines the Master desired would have to die. The Master began seeking out uncontaminated, pure-strain humans in New California, with whom he could create new Mutants and build his Super Mutant army.
However, creating new super mutants proved to be a hit-or-miss process. The great majority of super mutants produced by the Master, and later his lieutenant, in the Mariposa base's FEV vats were big, dumb brutes. Physically, they were vastly superior to pure-strain humans, but they possessed the intelligence of children. The Master was certain this mutational randomness was related to the amount of radiation damage suffered by an individual: humans who had not been exposed to much of the post-nuclear environment's radiation produced more intelligent Mutants after their FEV infection (see his personal diary for his thoughts on this). His lieutenant, however, had a different theory. When the Chinese nuclear bombs hit the original West Tek Research Facility and turned it into the region known only as the Glow, the impact of the nuclear shockwave cracked open a few nutrient solution tanks filled with FEV. The nuclear bombs' radiation then mutated this FEV into an airborne strain of the original virus, but this new airborne strain of the FEV did not have any real mutagenic effects on human beings. All it did was inoculate any person infected with it against the original, mutational strain of the FEV, acting as a form of vaccine and giving exposed creatures some limited resistance to the FEV's mutational effects. As a result, the ideal subject for FEV mutation was someone who had not been exposed to the airborne form of the FEV following the Great War. Both of these conditions - lack of radiation exposure and airborne FEV inoculation - were present in only one known population: Vault-dwellers. Each Vault contained around 1,000 viable subjects who could be infected with the original strain of the FEV and turned into intelligent super mutants. Which factor exactly determines what would emerge from an FEV infection - accumulated radiation levels or inoculation by the airborne FEV strain - is still uncertain, though as a rule of thumb, the less exposed the subject is to radiation, chemicals, and biological agents, the better.
The Enclave and the Modified FEV Strain
In 2236, the Enclave, the surviving remnant of the United States federal government, excavated the ruins of the Mariposa Military Base, largely using slave labor gathered from the town of Redding, and eventually discovered the remains of the Master's FEV vats deep inside the lower levels of the base. Though they had been buried in rock and forgotten for decades, the vats still contained that familiar, green, bubbling gunk, the nutrient solution that contained active particles of the Forced Evolutionary Virus. The Enclave gathered samples to be sent back to the oil rig that served as their headquarters off the Pacific coast of California for further analysis. Before long, though, the slave miners - some being fully exposed to raw FEV in the course of their labors, others likely falling into the FEV vats accidentally - began to mutate into a second generation of super mutants. The mutant slaves soon overthrew their Enclave guards. Seizing their weapons, hardware left behind by the first Mutant army or anything lying around the ruins that could serve as a weapon, the super mutants stormed the upper levels and quickly overran the Enclave troops in the base. Panicked and retreating, the Enclave troopers on the upper levels fled the base and used explosives to seal the entrance, trapping the second-generation Mutants inside the base.
Still, even though things went poorly at Mariposa, the Enclave had the samples of FEV its scientists needed. Back at the oil rig, the Enclave's United States Chemical Corps began work on the serum immediately. They found that it would be possible to create from the FEV an incredibly potent and lethal toxin. Dr. Charles Curling was the Enclave scientist who lead this project under the direction of Dick Richardson, the President of the United States. In order for the research to progress, however, the Enclave needed test subjects. They required two test groups: one of uncontaminated, pure-strain humans, and another composed of the "mutants" that lived all over the mainland of the West Coast, who had suffered from nuclear radiation and airborne FEV exposure for decades. To the Enclave, this exposure made all the peoples of the Wasteland genetically impure, regardless of how similar in appearance they were to pure-strain humans. The mutant sample was easily obtained when the Enclave's troops kidnapped the entire population of the town of Arroyo, who were all the descendants of citizens of Vault 13, to see if they had suffered mutation after they left the Vault.
The pure-strain human group proved difficult to obtain. The Enclave would not allow the Chemical Corps to use the Enclave's own people living on the oil rig. So, they had to look elsewhere. Once again, the mainland provided the needed stock. Thanks to the United States government's pre-War Vault Experiment project, the Enclave's scientists were able to determine that the pure-strain population of Vault 13 was largely intact. Thus, the Enclave's troops traveled in force to Vault 13 and sent a command to the Vault's computers to open the Vault door. After taking care of some mild resistance from the surprised Vault-dwellers, the Enclave's troops rounded up the residents of Vault 13 and shipped them off to the oil rig in Vertibirds to take part in the Chemical Corps' FEV experiments.
Before long, the Chemical Corps developed their viral supertoxin. It was found to be 100% lethal to humans who had suffered even mild radiation or airborne FEV exposure. Just as the Enclave was preparing to launch the poison into the North American jet stream to kill the entire population of the mainland, the Chosen One arrived at the oil rig on the Poseidon Oil tanker that had sat in the ruins of San Francisco for years. He freed the survivors of the Vault 13 population, as well as his own Arroyo tribals and started a self-destruct sequence on the oil rig that destroyed the entirety of the Enclave's headquarters, saving the surviving human population of the various American wastelands from a horrible death at the hands of the modified FEV strain - at least for a time.
The East Coast and FEV
In the Capital Wasteland, centered on the ruins of Washington, D.C. on the East Coast of what was the United States, yellow-skinned super mutants had also appeared along with a modified version of the chimaeric mutants known as centaurs. It was eventually learned that a modified version of the FEV was to blame for the emergence of this version of the super mutants and the centaurs on the East Coast. In the Capital Wasteland, Vault 87 had been equipped with a large supply of the FEV. In 2077, following the fall of the Chinese nuclear bombs on Washington, D.C., the dwellers of Vault 87 had been forcibly infected with a modified version of the FEV as a part of the secret Vault Experiment project controlled by the U.S. government and the Vault-Tec corporation. The modified version of the FEV used in Vault 87 gave the skin of the Super Mutants it created a yellow-green tint and made these mutants stronger and larger as they aged while greatly lowering their intelligence. This version of the virus also seemed to create more "healthy" super mutants than their New California counterparts, who were much more mutilated by the virus-induced mutations as a result of the Master's modifications. However, in the Capital Wasteland highly intelligent Super Mutants developed much less frequently, with only two "civilized" super mutants encountered in Fallout 3: Uncle Leo and Fawkes. Once in Vault 87, the Lone Wanderer can overhear two Super Mutants discussing collecting stronger men to make into stronger Super Mutants by putting them in "the green stuff". This is thought to be the ultimate intended fate of the super mutant captives strewn across the Capital Wasteland, and the reason why they are not simply killed or eaten. Uncle Leo also mentions that Super Mutants periodically abduct humans to create more super mutants using their diminishing supply of FEV located in Vault 87.
It is revealed during the quest Those! that while attempting to reduce the size of the ant populace of Capital Wasteland, Doctor Lesko lazily used the FEV (possibly a mutated version) as a shortcut while trying to modify the genes, and as a result of this shortcut, the ants ended up with the ability to breathe fire instead, which caused the deaths of everyone in Grayditch. The virus was found stored beside one of his terminals in Marigold Station, which was thought to be a controlled environment.
The Enclave's President John Henry Eden attempted to convince the Lone Wanderer to use the same modified form of FEV originally developed thirty years earlier by the Enclave on the West Coast to taint the activation of the Purifier. Like its predecessor this strain of the FEV had been created by the Enclave to kill all forms of life with any degree of radioactive or virally-induced mutation. If the Lone Wanderer carries out Eden's plan, then in the Broken Steel add-on the Wastes are devoid of those that had hoped to find new beginnings with the cleansing of the water of the Potomac River.
At least four different variations of the post-Great War FEV are known to exist, plus one rumored. The FEV is a major cause of mutations in all of the Fallout games, and usually is an instrument used by a villain to achieve their goals.
Mariposa FEV strain
The original strain of the virus that was developed at the Mariposa Military Base and later gave birth to the Master, Unity, and the army of the West Coast super mutants. All these super mutants were created using this strain, including Frank Horrigan, who was further augmented and experimented on by the Enclave's scientists. Other notable creatures created with this virus are centaurs, Floaters, and two ghoul-like mutants: Harold and Talius. The Master desired to use this virus to remake humanity into his own twisted vision of perfection. The Army had planned on releasing something similar to the super mutants in China before The Great War as seen in one Citadel terminal in Fallout 3. A number of remnant mutants from the original FEV experiments conducted by the Master have emigrated east and settled in places like the Mojave Wasteland.
Vault 87 FEV strain
This modified strain of FEV was located in Vault 87 in the Capital Wasteland and was also responsible for the creation of a super mutant army, this time without any clear leader. This strain completely wiped out any signs of pre-mutation gender and, compared to its western counterpart, produces quite different creatures who grow bigger as they age. This FEV strain is used by the super mutants to produce more of them in Vault 87. The design of Vault 87 indicates that this version of the FEV was administered in a gaseous state. This FEV strain, like the Mariposa version, gave birth to a group of misshapen mutants known as centaurs.
FEV Curling-13 strain
The Enclave, while led by President Dick Richardson, developed their own version of the FEV. It was an airborne modification that was intended to be spread by Vertibirds into the North American jet stream so that it could infect the entire American mainland. Created by the Enclave scientists led by Dr. Charles Curling, this strain of the virus was designed to kill all the "mutant" inhabitants of the post-nuclear United States.
Eden's FEV strain
When the Enclave relocated to the Capital Wasteland after the destruction of its oil rig headquarters off the coast of California in 2242, the survivors of the Oil Rig's destruction probably brought a sample of Richardson's modified Curling-13 strain of the FEV with them. President Richardson's successor as the leader of the Enclave, President John Henry Eden, a sentient artificial intelligence, personally remastered the virus, and altered it so that it was water-soluble instead of airborne. The President planned to use it to continue Richardson's plan to wipe the United States clean of all mutants, this time using Project Purity, an independent effort intended to cleanse the Potomac River of its radioactive contamination and provide clean water to the people of the Capital Wasteland. If you chose to infect Project Purity with this strain of FEV, Three Dog will report on how the people of the wastes are mysteriously becoming ill, but it only started when the purifier started 'chugging', he will say that 'the water is free, but not necessarily clean'.
Vault-Tec's FEV strain
|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
A secret Vault-Tec installation known as the Secret Vault was uncovered by the super mutant remnants of the Master's army, who were led by a super mutant named Attis. Attis revealed that Vault-Tec's scientists had intended to further improve the FEV. The super mutants saw the FEV as a chance to use as both a bio-weapon and as a key to humanity's evolutionary future, which is why they were determined to overcome the virus' primary disadvantage: the sterility it produced in mutants infected by it.
Attis proved unsuccessful in making FEV mutants fertile. However, the resulting Vault-Tec FEV strain provided mutants infected with new features such as faster cell regeneration and increased brain mass. The living bio-weapons created by this modified FEV included the unusual species of radbugs native to Texas, and the horrifying experimental deathclaws. When Attis, who was already transformed into a super mutant by that time used the modified strain on himself, he started to regenerate more rapidly and grew much larger; eventually becoming a giant, rapidly-multiplying, multi-cellular organism, which was destroyed by an explosion caused by the protagonist.
The FEV or its effects appear in some form in all Fallout games.
- ↑ ZAX.MSG: Lines 129 & 134 indicate ZAX 1.2 is fallible but intentionally so it could learn. Lines 135 & 154 are ZAX's defense of its authority on subject material. Line 134 also states certain functions are not subject to error but these functions are not specified.
- For biochemical description, see lines 157 & 166.
- Note: A protein sheathe reinforced with ionized Hydrogen would not come close to rendering something the size of a virus capable of absorbing or repelling a neutron (or gamma radiation). Instead, the virus would almost certainly be destroyed.
- For pre-programmed introns, see line 169.
- For exon copying and insertion, see lines 157 & 158.
- Note: The FEV most closely resembles a retrovirus which inserts RNA into a host cell which then uses reverse transcriptase to express the viral genes. The exons are described in correspondence to this resemblance but the introns are not.
- For recursive code, see lines 170, 172, & 173.
- For biochemical description, see lines 157 & 166.
- ↑ ZAX.MSG:
- For sterility details & confirmation, see Line 179
- For sterility confirmation, see Hypothesis paragraph 2, sentences 5 & 6.
- For potential sterility confirmation, see line 257.
- For cause of concern, see discourse of Lines 133, 135, 146, 149, & 153.
- For Gammorin's Army's interest, see Situation A01
- For bargaining chip, see Situation B02