Las Plagas (Spanish for "The Plagues") are the new agent of transformation in Resident Evil 4. Las Plagas are parasitic creatures that grow in the body of a human and control the behavior of the host. In-game reference material compares it to real parasites such as Toxoplasma gondii, Leucochloridium paradoxum and Cryptosporidium, all of which can influence the behavior of their host. Humans infested with Las Plagas remain mostly normal in appearance, but they exhibit heightened aggression and red irises that glow in low-light conditions. Their living conditions and hygiene have also deteriorated considerably.
The Plagas were sealed deep under the castle by the first castellan. This first castellan had taken away the rights of the early Los Illuminados ("The Enlightened Ones"). When the Plagas were unearthed, they seemed to have been fossilized. However, they had actually been in a dormant spore-like state. The first Ganados were those who had inhaled the spores while mining.
Las Plagas are extremely sensitive to light, only coming out of the host's body when it has sustained a lot of damage, or if its head is destroyed. Because of their sensitivity to light, Las Plagas can only reveal themselves at nightfall. The head of the host may either be destroyed completely and replaced by the parasite, or the parasite may emerge from the mouth or neck of the Ganado; however, due to their photosensitivity, Las Plagas can be destroyed with the use of a bright light source, such as a flash grenade.
Several varieties of Plaga may be seen during the course of the game. Some appear as the result of damage to certain parts of the host's body, such as the head, and may be a protrusion out of its host's body, or in whole as a detached entity. It is unclear if these are actually different varieties or mutations of Plaga or if they represent various levels of maturity. The following is a list of non-unique Plagas or their parts as seen in the game.
- Blade Plagas have a number of small tentacle-like appendages that swing erratically and are harmless, but one larger tentacle has a bone scythe at the end which is used to inflict damage by slashing at their enemy.
- Centipede Plagas attack by swallowing the head of their enemy whole.
- Spider Plagas are unique, in that they are capable of completely detaching from their host. They will eventually perish after detaching. While still attached they can spit acid at their enemy. They can also strike them with their legs, either while attached or separate.
- Tentacle-like protrusions, found in Colmillos. Unlike previous forms, they are not spawned as a result of damage, but rather are simply produced when the Colmillos has engaged in combat. They attack by whipping their enemy.
- Leech-like Plagas, most of which can only be seen using an infrared scope, are only found in Regenerators and Iron Maidens. They sustain the regenerative powers of both monsters.
Los Ganados (Spanish for "herd"; literally "the cattle") are the primary enemies in the game. Ganados are humans who have become infested by the mind-controlling Plaga parasites.
Three main types of Ganados are found in the game (by order of appearance): villagers, Zealots and militants.
VillagersAs revealed in the end credits, the villagers were once peaceful residents of a rural community until they were exposed to the parasites. From that point, they became ruthless zombie-like savages under Osmund Saddler's control. They continue to carry out agricultural duties, although their interest in basic hygiene has been lost and, subsequently, their living conditions have suffered. They wield farm tools as improvised weapons, including knives, axes, and sickles. When Leon first arrives in the village, he is immediately attacked by one of the villagers after questioning him about Ashley's whereabouts.
The Zealots are better equipped than the villagers, using a variety of medieval armaments. They wield weapons such as scythes, flails, crossbows, rocket launchers and wooden shields. All of them appear to be painted a pale white color with red streaks covering their face, resembling blood. In addition, all of them appear to be bald. Some have crimson lesions, tattoos or scarification from their rituals on their faces. Higher ranking Zealots wear a bullet-proof gold mask.
Ganados who wear old military uniforms and leather armor. Some wear brown boiler suits, gas masks, and carry stun rods.
The Garradors are Plaga-infested warriors who wear equipment resembling that worn by gladiators. The Garradors are equipped with large claws on each hand, which can be extended and retracted at will. The Garrador will indiscriminately attack anyone and anything in its path and are usually kept under restraint. Garradors are completely blind. They rely on their sense of hearing to track down intruders and are easily distracted by loud noises. Garradors will become enraged when very loud noises are made, and are prone to charging in the direction of the noise and striking at its source.
The word Garrador cannot be directly translated into English as no such word as Garrador exists in Spanish, although its substracted from desgarrador (ripper) ("garra" is Spanish for claw). The Garradors also appear in the Mercenaries minigame in the castle level, along with a Super Garrador, who is almost completely covered in armor, except the parasite.