The parasitic organism that infects Luis Serra in Resident Evil 4 is Las Plagas. Luis is a former Umbrella researcher who becomes involved with the parasite research and later helps Leon Kennedy understand the threat.
Las Plagas are parasitic organisms used by the Los Illuminados to control infected hosts. In the game, the parasites alter human behavior, grant unusual abilities, and can emerge from a host’s body in advanced stages. Luis’s scientific knowledge makes him a valuable source of information about the infection and its possible treatment.
Las Plagas are distinct from the T-virus and G-virus, which are viral agents from other Resident Evil storylines. They are also distinct from Uroboros, the bioweapon associated with Resident Evil 5. The parasite’s Spanish name means “the plagues,” matching the game’s rural European setting and the cult’s religious imagery.