The parasitic organism used by Los Illuminados in Resident Evil 4 is called Las Plagas.
Las Plagas are ancient parasites discovered beneath a Spanish castle. Los Illuminados revives and exploits them, using their ability to control infected hosts and coordinate their behavior.
Leon S. Kennedy encounters villagers and cult members whose actions are shaped by the parasites. The game distinguishes between different Plaga types, including forms associated with controlling hosts and producing severe mutations.
Las Plagas are not a virus, although they are part of Resident Evil’s broader bioweapon mythology. This distinction matters because the game’s enemies are not conventional reanimated zombies. Resident Evil 4’s parasite-based threat helped give the game a different atmosphere from the T-virus outbreak stories.