FOXDIE

"Liquid Snake: Fox...? Solid Snake: ...Die."

- Liquid Snake and Solid Snake prior to the former's death[src]

FOXDIE was an engineered retrovirus developed by the DIA for the Pentagon. It was programmed to kill specific people by identifying the person's DNA and their nanomachines, causing cardiac arrest.

Function
The FOXDIE virus enters macrophages in the target's bloodstream, then uses enzymes to attach to a DNA sequence that it has been "programmed" to recognize. Once the virus is attached to the DNA, the macrophage starts to produce the cytokine TNF (tumor necrosis factor) epsilon. The TNF-epsilon is carried along with the blood, to the heart, where it binds to the TNF receptors, causing the myocardial cells to die. This programmed cell death is called apoptosis, which leads to the victim suffering a heart attack. Natural heart attacks occur when the heart cells die from lack of oxygen and sugar, usually caused by a blocked coronary artery.

Origins
The development of the FOXDIE virus was one of the Pentagon's biological weapons programs. The FOXDIE project was led by the DIA operative Richard Ames under the authority of President George Sears, and under orders of the Patriots. The FOXDIE virus had been in development as the research hit a wall until 2003 when Dr. Naomi Hunter joined the project. When the virus was successfully engineered, it was Naomi who named it FOXDIE. Since then, the virus has been altered and re-created several times.

In February 2005, the Pentagon ordered Naomi to inject FOXDIE into an unassuming Solid Snake, shortly before his mission to Shadow Moses Island. The virus was programmed to kill the renegade members of FOXHOUND, who had taken over the island's nuclear disposal facility, as well as ArmsTech President Kenneth Baker. This was carried out in order to eliminate both the terrorists and anyone else involved on Shadow Moses, and to allow recovery of the hijacked Metal Gear REX, dummy warhead data, and the bodies of the Genome Soldiers, intact. Baker had some awareness of FOXDIE's development, but did not anticipate that it would actually be deployed. After becoming infected, Baker attempted to warn Snake of the virus upon deducing the cause of his sudden heart attack, but was unable to before drawing his last breath.

FOXHOUND commander Liquid Snake learned of FOXDIE's existence through a spy in the Pentagon, and added a FOXDIE vaccine to his list of demands to the U.S. government. Solid Snake eventually discovered that Naomi had made an alteration to FOXDIE so that it would also kill him, as an act of revenge for crippling her adoptive brother, Frank Jaeger.

Following Liquid's death from infection by the virus, Solid Snake inquired as to his own survival, given his similar genetic code, to which Naomi replied, "Live, Snake. That's all I can say to you." Since Naomi had set the virus's incubation period to a wildcard value, she was unaware of when exactly he would die, although FOXHOUND spy Revolver Ocelot (having himself survived due to access to the vaccine ) was under the impression that it would activate in Snake relatively soon after the Shadow Moses Incident. However, Snake would remain unaffected for years afterwards, since his and Liquid's DNA were too dissimilar to render them both as identical targets.

Post-Shadow Moses
Nastasha Romanenko received an optical disc, containing all the records of the personnel and research pertaining to the FOXDIE project, from her ex-husband Ames. He willingly gave it to her in an attempt to keep her safe from the Patriots by using the disc's contents as leverage in case they tried to harm her. Shortly thereafter, she used the contents of the disc to explain the true events of the Shadow Moses Incident in a book called In the Darkness of Shadow Moses: The Unofficial Truth.

FOXDIE was briefly referenced by Snake during the Tanker Incident in 2007, when he stated that he wouldn't have to worry about being given any "unwanted gifts" while operating as part of Philanthropy. In 2009, a digital version of FOXDIE was uploaded into the GW AI of Arsenal Gear. Designed by Emma Emmerich to disrupt GW's operating system and render it unusable in case it fell into the wrong hands, it was altered just before the incident by the Patriots to delete any data about their "true" identities. The computer virus was part of the S3 Plan that manipulated the events of the Big Shell Incident into an orchestrated recreation of the Shadow Moses Incident. Besides the worm cluster, the Patriots also remotely deactivated Ames' (ironically the leader of the FOXDIE project) nanomachines when Ocelot discovered him, in order to ensure that he died in a similar manner to Decoy Octopus and Kenneth Baker before him.

Post-Big Shell
After the SOP System was developed, it incorporated the nanomachine base for FOXDIE, namely as a strict genetic lock for the System to prevent it from being hacked.

The biological FOXDIE that was still in Old Snake began to mutate when his body started to age at an accelerated rate. This new FOXDIE would eventually lose its ability to kill based on specific DNA sequences and would begin to kill people at random, possibly becoming a worldwide pandemic. Snake learned of this after Naomi Hunter conducted medical tests on him in 2014.

It later became apparent, however, that Snake had previously been injected with a newer form of FOXDIE by Drebin during his failed mission to assassinate Liquid Ocelot in the Middle East, which halted the mutation of the original virus. The new FOXDIE was programmed by the Patriots to kill Big Mama, Liquid Ocelot and Big Boss. Although the new FOXDIE had uprooted the old, it would eventually mutate as well and become dangerous to the general population. However, this would not occur for several years, and would only live as long as Snake, which was an estimated year at best.

Other than the mutated and new strands of FOXDIE, Liquid Ocelot briefly alluded to his earlier death from FOXDIE shortly after being defeated in a Metal Gear battle with Snake on Shadow Moses, including re-enacting how he had earlier died, albeit with the phrases used reversed. However, Liquid then got back up and revealed he had been faking before telling Snake that FOXDIE "won't work on him this time."

Victims of the old FOXDIE

 * Decoy Octopus
 * Kenneth Baker
 * Liquid Snake

Victims of the new FOXDIE

 * EVA
 * Liquid/Revolver Ocelot
 * Big Boss

Behind the scenes
The original FOXDIE virus appears in Metal Gear Solid. In the English version, its name is rendered as "FoxDie," whereas subsequent games and sources give the whole name in upper case.

In Metal Gear Solid: The Twin Snakes, Revolver Ocelot claims that he survived the events on Shadow Moses due to the FOXDIE vaccine, in his conversation with Solidus Snake following the game's end credits, although the Metal Gear Solid 4 Database states that it is unclear whether or not it ever actually existed. Both the Database and Naomi Hunter's character profile on the official Metal Gear Solid 4 website mistakenly identify the CIA as FOXDIE's developers, rather than the DIA.

According to Hideo Kojima's "Grand Game Plan" for Metal Gear Solid 2, it was intended that Solidus would be a target for Snake's FOXDIE by the Patriots, but was immune due to having received the vaccine.

In Metal Gear Solid 4: Guns of the Patriots, Ocelot did not succumb to FOXDIE while in his Liquid persona, despite having been in close proximity to Solid Snake during Act 3. The virus only started to take effect when his original personality resurfaced in Act 5.

In the Déjà Vu mission for Metal Gear Solid V: Ground Zeroes, when Classic Snake/Big Boss arrives at the animal cage where Metal Gear Solid-era Donald Anderson is being held, the latter starts experiencing convulsions mirroring that of FOXDIE while also yelling "FOXDIE!". Finding Kenneth Baker at the admin building's boiler room will have the same effects. Note that they only appear as Anderson and Baker, respectively, if the player is playing with the Classic Snake or Cyborg Ninja skin. Upon encountering both POWs and witnessing their deaths, the player will end up "infected" with FOXDIE, causing the game to glitch up and experience psychedelic colors with computer code in the background before being "rebooted" by the Fox Engine. In addition, after completing all the quiz questions post-mission, Liquid will end up succumbing to FOXDIE, with the player having to complete Liquid's statement as the "eleventh" question.

Appearances

 * Metal Gear Solid
 * Metal Gear Solid 2: Sons of Liberty (mentioned)
 * Metal Gear Solid: The Twin Snakes
 * Metal Gear Solid 4: Guns of the Patriots
 * Metal Gear Solid V: Ground Zeroes (mentioned in the Déjà Vu mission; non-canon)