What Is FALLOUT’s Forced Evolutionary Virus? The FEV, Explained

Fallout season two, episode five drops a massive bombshell into the world of the series. As Norm MacLean continues to lead Bud’s Buds into the brave new post-Great War world, he comes across a massive revelation, one that Fallout season two was already hinting at. In episode four of the show’s second chapter, Bud’s personal assistant, Ronnie McCurtry, intimated that he knew all about the “whole plan” for Vault-Tec’s Vaults 32 and 33, the one called “Future Enterprise Ventures,” or FEV. Of course, fans of Fallout will know that FEV stands for something entirely different in the franchise: the Forced Evolutionary Virus. And Fallout season two, episode five, confirmed that’s what Vault-Tec’s plan was all about. When Norm powers up an old Vault-Tec computer, he finds what we suspected, that Ronnie’s FEV acronym points to Vault-Tec messing with the Forced Evolutionary Virus. But what is Fallout‘s Forced Evolutionary Virus? And what does it do? Join us as we delve deep into the lore and explain everything you need to know about the FEV, or Forced Evolutionary Virus, in Fallout. Buckle up, Vaulties, this one is complicated.

What Is Fallout’s Forced Evolutionary Virus?

Fallout‘s Forced Evolutionary Virus, or FEV, is an artificial megavirus created by West-Tek’s NBC Division in the mid-2070s. It’s transmissible by injection, direct contact, or an aerosol solution. The purpose of this Fallout virus is to alter the DNA of the host and “evolve” their genetic makeup in a variety of ways.

The Forced Evolutionary Virus is immune to radiation, making it even more viable in a post-war world. The virus is pre-programmed with “corrected” DNA patterns appropriate to a species, which is merged with the FEV’s unique recursive code. Well, appropriate to a species, in theory. Most uses of Forced Evolutionary Virus on Fallout weren’t appropriate in any sense. Initially, the Forced Evolutionary Virus began as an attempt to cure a different disease, but over time, it just became a way to try to create genetically enhanced Super Soldiers.

When the Forced Evolutionary Virus is administered to a genetically viable individual in Fallout‘s world, the host’s DNA is completely rewritten. The process is usually extremely painful when delivered in excess, leaving lasting mental damage. FEV delivered via aerosol over a long period of time usually has less lethal effects on the victim. In non-viable individuals, such as those with pre-existing radiation damage, the result is typically massive organ failure and subsequent death. 

The ease of modifying the Forced Evolutionary Virus for whatever “evolution” of genetics is desired has not only made it one of the most powerful biological weapons to ever exist, but also a threat that has come up multiple times in the Fallout games, each one a bit different than the other. FEV is often pictured as green goo in giant vats in Fallout‘s games. 

So…What Does Fallout‘s Forced Evolutionary Virus Do To You? What Are the FEV’s Symptoms and Outcomes?

Let’s brass tacks and dig ourselves out of this science mumbo-jumbo for a moment. Simply put, this stuff really messes you up. I think one of the best examples of what Fallout‘s Forced Evolutionary Virus does is in Vault 87 in Fallout 3. Through the physical evidence left behind, terminal entries, and the first-hand account given to you by Fawkes (the only Super Mutant in Vault 87 to retain his full mental capabilities), you can follow the horror step-by-step down the rabbithole—at least for this strain of FEV. 

The FEV Can Create Super Mutants in Fallout

After seeking the help of the denizens of Little Lamplight (who are all children, long story), the Lone Wanderer can enter the blood-drenched, shadowy confines of Vault 87. This is the stronghold of the Super Mutants and Centaurs that haunt the Capital Wasteland. Vault 87, and everything inside of it, is a testament to man’s inability to resist the march of progress, even if bones are breaking under their feet. After the initial social experiment of Vault 87 fell through, it was turned into a secondary research facility studying the effects of FEV, called the Evolutionary Experimentation Program. Let me walk you through the journey of their test subjects. 

Post-FEV infection, visible sex markers began to disappear. Diversity win: the Forced Evolutionary Virus killing you is agender! Within ten days, the skin of the subjects became resilient and much thicker. In this timeframe, male subjects usually developed advanced upper-body strength as well. Within a timeframe of two weeks, the subject becomes increasingly angry and increasingly anxious, leading to violent outbursts. After a female test subject died due to massive loss of her brain function, this Fallout FEV experiment was called off, and all the other subjects were terminated. 

The FEV Super Mutants Escaped Containment

Like most Vault-Tec experiments, it wasn’t called off quite as much as it should have been, though, and eventually some of the Forced Evolutionary Virus mutants overwhelmed Vault-Tec security forces and broke containment, hence why this yellow-green variant of Super Mutants can be found all over the Capital Wasteland in Fallout. In order to perpetuate their race, as they can’t reproduce, Super Mutants often take human captives back to Vault 87, to force their ‘evolution’ as well using FEV. 

Per Uncle Leo, one of Fallout 3’s two friendly Super Mutants: “Maybe you don’t know where we come from. Super Mutants, I mean. We aren’t born this way. We are created in the cold, dark, metal place, where my brothers bring their captives, endlessly trying to make more of us. It was something I could not do. I tried to get them to stop, but they wouldn’t listen. They would have killed me if I had not run away.”

Other Symptoms and Consequences of the Forced Evolutionary Virus: Death and Worse Fates

While Super Mutants are the most common human + FEV result, they’re not the only ones. Obviously, straight-up dropping dead is always a possibility. There are failed Forced Evolutionary Virus experiments in Fallout 3‘s Vault 87 to be found, and they look like Cronenberg-esque nightmare fuel. The muscle growth is consistent with Super Mutants, but seemingly random—the muscles and bones are bursting through the skin. They died in horrific pain, as their faces show when you look upon their pitiable forms.

Then there’s the Centaurs, and boy-howdy, they’re even worse. They’re alive, for one thing. The east coast variants created in Vault 87 are one-headed Centaurs, as opposed to their west coast counterparts, who are two-headed. Fallout‘s FEV-created Centaurs can’t speak, as their mouth is full of tentacle-like tongues that they use to lash enemies. A muscular human-ish torso sits upright, while the lower body is sloped and insect-like, its belly dragging on the ground. This Fallout creature moves itself with four arms, two in the front, and two in the back. The pair up front pulls the creature forward, and the back arm-leg-things drag along the ground next to the abdomen. 

The Forced Evolutionary Virus-created centaurs serve as watchdogs to the Super Mutants. It’s unclear how much intelligence they’ve retained, but for their sake, I hope it isn’t much.

We Need to Talk About Harold

So, there’s a character that appeared in Fallout 1, Fallout 2, and Fallout 3. His name is Harold. Harold has a friend named Bob, a little plant that’s growing out of his head in the first game. By the third game, Harold is a goddamn tree begging for death. 

In Fallout 3, the Lone Wanderer can meet Harold if they can find their way into Oasis, a secluded, secret community that lives in a thriving and vibrant grove, filled with plant life. It is a complete contrast to the rest of the wastes, especially the Capital Wasteland. DC and the surrounding environs were utterly destroyed by the bombs, leaving little left but the occasional brown shrub and gray ghosts of trees long-dead. And it turns out, this lush grove all came from Harold’s Forced Evolutionary Virus mutation.

Just another example of what FEV can do to a body on Fallout—the menagerie of mutation never ends!

Does The Forced Evolutionary Virus Have a Cure on Fallout?

To date, there is no cure for the Forced Evolutionary Virus on Fallout. Individual cases and strains of the FEV can be cured, reversed, or mitigated, as we see in Fallout 4 with Virgil, which we’ll get into below. The Enclave was also able to create an aerosol inoculation, which worked to prevent its specific strain of FEV from infecting a subject. But again, this was more like a vaccine than a cure, and linked to one specific kind of FEV. No overall cure that we know of exists in Fallout‘s world at this time.

Theoretically, someone infected with the Forced Evolutionary Virus could be “cured” by a virus that re-infects the subject with their original DNA. But this would not be a total cure, but another mutation of the body.

The Origins of the Forced Evolutionary Virus in Fallout and Its Pre-War Strains

The FEV Started as a Cure for the New Plauge on Fallout

Believe it or not, the Forced Evolutionary Virus started with good intentions in Fallout‘s world. In the 2050s, the New Plague—a virulent and deadly virus—tore through America. In 2055, West-Tek’s NBC division, with the help of a ZAX 1.2 supercomputer, began research on a cure. They were left to their own devices until 2073, when the United States government was intrigued by the research and took over the project. 

The government then created the Pan-Virion Immunity Project, in the hopes of countering both the New Plague virus and any possible biological threats from enemies, foreign and domestic. This holotape entry from a scientist who worked on the PVP project can be found in Fallout 1—

“It became clear that the best way to combat the newly created biological weapons was to alter uninfected DNA so that it was no longer susceptible to standard viral infection.”

Good Intentions Quickly Went Bad as the Pan-Virion Immunity Project Became the Forced Evolutionary Virus

In 2075, Batch 10-011 Pan-Immunity Variant was created. Not long after, the Pan-Immunity Variant’s name was changed to the Forced Evolutionary Virus. The government started right in with animal experimentation to test the FEV, beginning with flatworms. The worms displayed resistance to over forty different viral contagions after being exposed to the Forced Evolutionary Virus. It was noted that their body masses also increased due to the FEV exposure. 

Eventually, the FEV (let’s call this unnamed strain FEV-1) was tested on mice, rabbits, molerats, raccoons, dogs, and chimpanzees. Each trial yielded pretty much the same result: increased strength, speed, intelligence, and aggression. Once the military caught wind of this, they took the FEV-1 for their own, and moved operations to the newly constructed Mariposa Military Base to further develop the virus. The new strain would come to be known as FEV-2. 

Before the Great War, The Military Exposed Humans to FEV on Fallout For Its Own Aims

It didn’t take long for the military to start using Fallout‘s FEV to experiment on human ‘volunteers’. When the failed experiments were discovered by the army personnel guarding the base, Captain Roger Maxson and his unit deserted the United States military and formed the Brotherhood of Steel, intent on the containment of technology—to make sure that something like this never happened again. (By the way, it totally happened again.)

During the timeframe of the FEV-2 experiments on the West Coast, the East Coast had its own burgeoning Forced Evolutionary Virus plots to contend with in Fallout‘s world. The military struck up a deal with the dying town of Huntersville, West Virginia. The military offers Huntersville new sewers and a water recycling system, in exchange for being able to build a West-Tek facility near the town. The research was purportedly supposed to be about crops, but before they even began, a new directive was given: continue to work on FEV and test it on the townspeople of Huntersville.

These Experiments with the Forced Evolutionary Virus Go Wrong Quickly

You’ll be shocked to hear that people started getting pretty sick en masse not long after. The locals pleaded with West-Tek for any support they could provide. West-Tek told them they just had an uncommon strain of the flu. But things got worse. People started to turn green and lose their hair. Then they began to mutate in horrific ways. The army’s 12th Mountain Regiment was even called in to better keep the FEV test subjects from escaping. Fighting broke out soon after. 

The West-Tek in West Virgina manages to create some truly terrible things. Though it took a tremendous amount of effort, they eventually made something biologically stable with the Forced Evolutionary Virus. Their first FEV creations are called Snally-gasters by the post-war denizens of Appalachia. They’re common enemies in Fallout 76. The Snally-gasters are awful little monsters with too many limbs and eyes.

But, impressively, the West-Tek operation birthed something even worse into Fallout‘s world using the Forced Evolutionary Virus. They managed to cook up a humongous monster. Of course, the monster broke containment. The military was sent after the creature and were able to subdue it. Unfortunately, on the way back to Huntersville, the bombs dropped and Fallout‘s Great War began. In the chaos, the experiment escaped. In Fallout 76, this FEV creature is referred to as the Grafton Monster. It’s basically a headless, hairless creature that spews corrosive, oily tar. Thanks, Forced Evolutionary Virus. 

What Is the Purpose of the Forced Evolutionary Virus In Fallout‘s Post-War World

It will not surprise you to hear that the purpose of Fallout‘s Forced Evolutionary Virus is mostly super soldiers. 

The Forced Evolutionary Virus in Fallout 1

In Fallout 1, the Master of Super Mutants (usually just called ‘The Master’) uses FEV to create an army of loyal Super Mutants that he calls The Unity. The Master’s goal was to turn the entire human race into Super Mutants. To create one single, perfect race, in the name of lasting peace. Nothing says peace like science crimes and murder!

The Forced Evolutionary Virus in Fallout 2

In Fallout 2, a new Forced Evolutionary Virus strain is born. The Enclave (one of Fallout‘s most villainous organizations that that is an  anti-communist, paramilitary organization) has a strain of FEV-2 that only bonds with human glyco-proteins. The Enclave used that functionality to turn it into FEV Curling-13, a potent toxin that specifically targets humans, mutated or not. Meanwhile, the Enclave’s Chemical Corps was using the Forced Evolutionary Virus to create Frank Horrigan, the main villain in Fallout 2. Frank Horrigan is a super mutant that retained his intelligence and was a member of the Enclave’s secret service in 2241 after years of surgeries to make him even more powerful. This was overseen by Colonel Dr. Charles Curling, the namesake of the FEV Curling-13 strain. 

The Forced Evolutionary Virus in Fallout 3

Towards the tail-end of the main campaign in Fallout 3, you meet President John Henry Eden, a ZAX supercomputer in the hands of the Enclave that believes he is the president of the United States. I guess no one else was running for the spot. When he meets the Lone Wanderer in Fallout 3, President Eden makes them take a vial of one of the Enclave’s multiple Forced Evolutionary Virus strains. Eden wants the player to put the FEV strain into the water purifier at the Jefferson Memorial. 

The intention is to eradicate mutation in the wasteland… By killing everything that’s got genetic damage from radiation. Which…pretty much means everyone but Vaulties and, you guessed it—the Enclave itself. President Eden tries to sugarcoat it, but this intended use of the Forced Evolutionary Virus in Fallout 3 genocide, pure and simple. 

The Forced Evolutionary Virus in Fallout 4

Fallout 4 has its own FEV strains, too, of course. In the 2100s, the Institute (a reclusive scientific community in Fallout‘s world) got ahold of the Forced Evolutionary Virus, and they’ve been having fun with it ever since. Players were initially puzzled by the presence of Super Mutants in the Commonwealth, especially since they didn’t look much like the Super Mutants in DC—and the California mutants certainly didn’t reverse Manifest Destiny themselves across the country.

Surprise! The Institute made these Super Mutants with the Forced Evolutionary Virus in Fallout 4. The Sole Survivor can find the old FEV lab within the Institute, abandoned and shuttered in the wake of Dr. Brian Virgil’s betrayal. Virgil sabotaged the operation, then turned himself into a Super Mutant and fled into the Glowing Sea, a highly irradiated place that the Institute could not follow him to. The Sole Survivor can sneak into the lab and steal a serum that cures Virgil’s Super Mutant-ness, the first of its kind seen in the games. It works too! Give it to Virgil, come back later, and boom! Human!

The Forced Evolutionary Virus in Prime Video’s Fallout Series

Alright, take a breath everyone, that’s the end of the game horrors. But now we get SHOW horrors! Even…better? When Norm hacks into Barb’s computer in Fallout season two, episode five, he finds an entry on the FEV, or Forced Evolutionary Virus. This is the first time the show has said it outright. Also, calling it now— that was Forced Evolutionary Virus that Dr. Chickenlover/the Snakeoil Salesman gave to Thaddeus last season on Fallout. So, no, I don’t think Thaddeus is turning into a ghoul. I think Thaddeus is turning into a Super Mutant on Fallout season two. I’ve never heard of FEV creating ghouls in any of Fallout‘s games. 

Fallout Season 2 Confirms Vault-Tec Is Planning to Use the FEV on Their Vaults

All of this begs the question: what is Vault-Tec doing with FEV? If Barb knows about it, then something very sketchy is going down. We know from the games that Vault-Tec’s Vault 87 was turned into a Forced Evolutionary Virus research facility, but I think the show might take it a step further. Now that they’ve pulled this particularly long plot thread from the games, there’s no real telling where it will go. 

But we do know from good old Ronnie McCurtry that FEV plays a major role in “Phase 2” of Bud and Vault-Tec’s “whole plan for Vaults 32 and 33.” But also, thanks to Ronnie, we and Norm don’t get to read what was written about the Forced Evolutionary Viruson Barb’s computer screen in Fallout season two. All we see is that “F.E.V. (Forced Evolutionary Virus) is the gene-altering agent for organism supercharging. The following is a…” And cut to black.

Could The Fallout Series Confirm that the Forced Evolutionary Virus Was Used to Create Deathclaws?

That pre-war Deathclaw in Fallout season two, episode four sure did seem like it was on Cooper’s side, didn’t it? In the Fallout games, the United States develops the Deathclaws, and unless I’m missing something, it’s never stated whether the Forced Evolutionary Virus was used in their initial creation. Of course, we know the Master absolutely used FEV to refine Deathclaws into the delightful killing machines we know today. 

Ultimately, we can’t wait to see how the show ties these plot threads together. The Forced Evolutionary Virus has been a core lore element of the Fallout games since 1997, and it only makes sense that it would make it to the TV series. 

Fallout season two airs Wednesdays on Prime Video.