FOXDIE

FOXDIE is an engineered retrovirus developed by the Pentagon. It is programmed to kill specific people by identifying the person's DNA and their nanomachines. Afterwards, it induces cardiac arrest, killing the target.

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 program was led by Richard Ames under the authority of President George Sears. The FOXDIE virus had been in development as the research hit a wall until 2003 when Dr. Naomi Hunter joined the project. Since then, the virus has been altered and re-created several times.

In 2005, the Pentagon had the US Secretary of Defense Jim Houseman give the order for Dr. Hunter to inject the virus into an unaware Solid Snake shortly before he departed for Shadow Moses Island. The virus was programmed to kill off the ArmsTech President Kenneth Baker and the renegade members of FOXHOUND. The reason they injected the virus into Snake is so he could take down both the Sons of Big Boss and anyone else involved on Shadow Moses and at the same time the Pentagon would recover Metal Gear REX, the dummy warhead data, and the bodies of the Genome Soldiers intact.

Snake eventually discovered that Naomi had made an alteration to the virus so that at an unknown point in time it would also kill Snake, as an act of revenge for "killing" her closest relative: Frank Jaeger. When asked by Snake, regarding the amount of time he had until the virus killed him, Naomi replied, "Live, Snake. That's all I can say to you." Since Naomi had set the virus to activate at a "wildcard" value, she was unaware of when exactly he would die. However, it was later revealed that this FOXDIE wouldn't kill Snake due to unforeseen technicalities in Snake's DNA, due to the Les Enfants Terribles project.

During the events of the Tanker Incident in 2007, Solid Snake briefly referenced Naomi's injection of FOXDIE into him when remarking that he at least won't be given any more "unwanted gifts" while doing 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.

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.

Unknown to Snake, however, he had previously been injected with a newer form of FOXDIE by Drebin 893, 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.