In Metal Gear Solid, Solid Snake carries the FOXDIE virus, which is designed to kill selected genetic targets.
FOXDIE is a genetically engineered biological weapon implanted in Snake before the Shadow Moses mission. It activates when its programmed target-recognition conditions are met, allowing it to attack specific people rather than spreading indiscriminately like a conventional infection.
The virus becomes central to the game’s ending because several people connected to the mission die after exposure, while Snake’s own future remains uncertain. The story uses FOXDIE to connect genetics, military control, and the hidden purpose of Snake’s assignment.
FOXDIE should not be confused with the genome soldiers’ genetic enhancements or the vocal cord parasites introduced in later Metal Gear games. Those are separate biological technologies with different mechanisms and story roles.