Las Plagas is the parasite controlling the Ganados in Resident Evil 4.
Las Plagas are parasitic organisms used by the Los Illuminados cult to control infected hosts. In Resident Evil 4, Leon S. Kennedy travels to a rural region of Spain to rescue Ashley Graham and encounters villagers known as Ganados, many of whom are infected with the parasite.
Unlike the shambling zombies associated with the T-virus, Ganados can move quickly, use weapons, and communicate. This change helped Resident Evil 4 emphasize aggressive crowd encounters and precise combat. The parasite can also emerge from hosts in more advanced forms, producing dangerous mutations.
Las Plagas is a plural name, although players often use it as a general label for the parasite outbreak. It should not be confused with Uroboros from Resident Evil 5 or the Mold from Resident Evil 7. Those are separate biological threats in the series.