FEV: the Forced Evolutionary Virus in Fallout Explained

FEV, the Forced Evolutionary Virus, is a man-made virus that rewrites the DNA of anything it infects. The pre-war contractor West Tek developed it for the US military, and after the Great War it became the source of Fallout's super mutants. The Master used it to build an army, the Enclave turned it into a plague meant to wipe out everyone outside its ranks, and the Institute used it on kidnapped wastelanders. (Fallout; Fallout 2; Fallout 3; Fallout 4)

Who made FEV, and why#

FEV came out of West Tek's research labs in California, as the original Fallout reveals. The facility's computer, ZAX, tells the Vault Dweller that research into a cure for the New Plague produced the virus, and that the team then pushed it towards something bigger: a transmissible genetic-engineering virus, or infectious evolution. (Fallout)

The research logs at the ruined West Tek site, later called the Glow, record the tests in order. Flatworms grew larger and fought off dozens of other infections. Mice grew bigger and solved mazes faster. Raccoons got cleverer and more dextrous, and some escaped their cages. Dogs grew fast and aggressive. Chimpanzees became huge and seemingly immune to cancer and chemical agents, but some turned violent and a few died from seizures. (Fallout)

Then the military took over. The project moved to the newly built Mariposa Military Base, and testing began on humans despite objections from West Tek staff. (Fallout)

Mariposa and the birth of the Brotherhood#

Captain Roger Maxson commanded the soldiers guarding Mariposa. His diary, which the Vault Dweller can read in the original game, records the moment his commander found out the scientists were using military prisoners as test subjects and broke down. Maxson took over, interrogated the scientists, and had them executed. His unit then deserted, and that group became the Brotherhood of Steel. (Fallout)

The TV series puts a face on the science side. West Tek researcher Leon Von Felden sits at the Vault-Tec summit in the season 1 finale, and in season 2 Norm MacLean, searching a computer in the ruins of Vault-Tec's headquarters, finds that a project filed as "Future Enterprise Ventures" is really the Forced Evolutionary Virus. (Fallout, "The Beginning"; Fallout, "The Wrangler")

How FEV works#

ZAX explains it in the original game. The virus copies a host's DNA, merges it with the corrected patterns it carries, and reinjects the result, so the host is rebuilt to the virus's design. Because some of the host's own code goes back in, the results can be unpredictable. It's spread by contact or injection, not through the air from one person to the next, and its structure makes it immune to radiation. (Fallout)

How much you get matters. The Master's method was to submerge captured humans in vats of FEV for a full dose. People already damaged by radiation tended to die or turn out badly, which is why he hunted for clean subjects from the Vaults. (Fallout)

A Brotherhood autopsy of a super mutant in Fallout lists what the virus does to a human: well over three metres tall, mostly muscle, tough dry skin, faster cell division, no inherited diseases. It also wipes out the ability to reproduce, a fact the Vault Dweller can use against the Master. (Fallout)

Every major FEV strain#

• The Master's FEV. Richard Grey fell into a vat at Mariposa during an expedition and fused into something new. As the Master he dipped thousands of humans to build the Unity, a mutant army he thought would end humanity's conflicts. The Vault Dweller destroyed the vats and the Master. (Fallout)
• FEV Curling-13. The Enclave dug up the Mariposa virus and had its Chemical Corps turn it into an airborne toxin that would bond only with human proteins and kill every person who hadn't been given the Enclave's inoculation. It was tested on the captured people of Vault 13 and Arroyo, and the Chosen One stopped its release from the oil rig. (Fallout 2)
• Vault 87's strain. Vault-Tec gave this East Coast Vault its own FEV experiment. Its dwellers became the dim, violent super mutants who overrun Washington, D.C. (Fallout 3)
• Eden's modified FEV. President John Henry Eden of the Enclave wants the Lone Wanderer to put a modified strain into Project Purity's water, killing every mutated person in the Capital Wasteland. If it's used, Broken Steel shows the sick and dying in every settlement. (Fallout 3; Fallout 3: Broken Steel)
• The Institute's strain. In Boston, the Institute experimented with FEV on people it abducted from the surface, which is where the Commonwealth's super mutants came from. Its research also fed into the human-like synths. (Fallout 4)

Can FEV be cured?#

For most of the series, no. ZAX tells the Vault Dweller that undoing it would take a second virus to put the original DNA back, a second mutation rather than a cure. The Enclave developed an inoculation, but it only protects against Curling-13. (Fallout; Fallout 2)

The nearest thing to a cure is Brian Virgil's serum in Fallout 4. A former Institute scientist who turned himself into a super mutant to escape, Virgil can be given back the serum he was working on, and it reverses his own change. (Fallout 4)

Did FEV start the Great War?#

The games stop short of saying so. Tim Cain, co-creator of the original Fallout, said in a 2023 interview that China launched because it found out the US was developing FEV, an illegal bio-weapon, and the US only pretended to stop by moving the research. For more on that argument, see the Great War. (Tim Cain on who started the Great War)

The virus also shaped the post-war world in a quieter way. The Brotherhood's habit of hoarding dangerous technology goes back to Maxson's discovery at Mariposa, and Brotherhood members in Fallout 4 still point to FEV as proof of what happens when science goes unchecked. (Fallout; Fallout 4)