Plaga

Las Plagas (from Spanish, meaning The Plagues) are a breed of parasitic organisms and the central focus of Resident Evil 4 's storyline. Unlike the T-Virus, which turned its victims into mindless zombies, the victims of Las Plagas (such as the Ganados) are violent savages that retain their intelligence and work collectively to pursue their target.

History
Las Plagas were first discovered by a religious group in Europe known as "Los Illuminados" in a unknown region of Spain. The cult used the Plagas as a mean of recruiting new members. However, the first castellan of the Salazar family opposed the cult and sealed off access to the parasites in his castle.

Centuries later, Ramon Salazar, the eighth Salazar castellan, was brainwashed by Osmund Saddler, the new leader of Los Illuminados, into joining Los Illuminados and opening the seal that concealed the Plagas. The two hired the local villagers as miners to undo the seal.

Due to years of concealment, the Plagas became fossilized in the rocks below Salazar's castle. However, the Plagas were still alive at the microscopic level in spores within the fossils. The miners that were excavating the site accidentally inhaled the spores while excavating which caused the revival of the parasites within their bodies. Since then, Saddler developed Plagas eggs by growing spores and having them mature, and then injecting his followers with the parasite.

Albert Wesker, who is now apparently running the Umbrella Corporation which had gone underground after the events of Raccoon City, had tasked Ada Wong with retrieving a sample of the Plagas in order to help reestablish Umbrella after learning of the creature by messages sent by Luis Sera that were intercepted. He previously had sent in a new operative, Jack Krauser, to get inside Saddler's inner circle but later sent in Ada after it appeared Saddler did not trust Krauser. Ada ended up betraying Wesker unbeknownst to him in the game, as she gave the Plaga sample she stole to a group she refers to as "the Agency," probably the same one that Wesker was working for but probably left to be the new boss of the fallen Umbrella corp.

Characteristics
Las Plagas is a type of parasite which alters its host's behaviour by attaching itself internally into the nervous system. The undeveloped parasite is found between the lungs, beneath the heart, where it can tap into the host's spinal cord, which is a link to the rest of the nervous system. Unlike other parasites, the Plagas are a social organism, meaning that they live together in social harmony (instead of individually). Luis Sera postulates that they have a collective intelligence. It is also believed that the parasites can communicate with each other through a unique sonic wave frequency that can only be heard by them, although this is purely hypothetical. Nevertheless, the Ganados appeared to be under the influence of Saddler, who used a ceremonial rod made out of a Plaga. The loyalty of a parasite depends on the host.

Besides influencing the actions of those they infect, Plagas leave several distinct side effects. These include, but are not limited to: coughing up blood, convulsions, sudden unconsciousness, hallucinations, veins and arteries show throughout skin and discoloration of the iris – usually to a reddish color.

The parasites also have a high adaptability rate, as they can possess other kinds of organisms besides humans. (Ramon Salazar uses Plagas-infected wolves as guard dogs in his castle.) In addition to the Ganados, Saddler and his men also created several kinds of mutant creatures with Las Plagas such as El Gigante, the Regenerators, Novistadors, and the U3 as a result of genetic experimentation with the parasites and human specimens.

All of the members of Los Illuminados, including Saddler himself, have Plaga parasites implanted into their bodies. However, the ones used by him and his top underlings (Bitores Mendez and Salazar) are that of "queen plagas" that can control the plagas through sonic signals, thus controlling the Plagas which control the humans.

Except for manifesting when sufficient trauma is induced on a host, Plagas have no true mutagenic effect on their hosts' body. While the Plaga infecting the Ganados and Colmillos can manifest and mutilate the body of their host, they do not actually induce any significant mutation or physiological change like Umbrella's myriad viruses do. Other creatures like the Novistadors and Garradors do exhibit mutation, but this is the result of actual genetic manipulation, not an effect of casual infection. However, it is the fact that victims exhibit no outward signs of infestation besides reddish-discolored irises which makes the Plagas a very dangerous threat, since it is much more subtle and invasive than Umbrella's viruses.

Countermeasures
If a person has been infected with the parasite, then the parasite can still be neutralized with medication before the egg hatches. However, once the Plaga has hatched, then the only way to get rid of the parasite is by undergoing a special kind of radiation treatment (specialized pills can be used to suppress growth of the parasites until then). This extremely painful procedure has the possibility of impairing the host's consciousness, and may kill the host if the Plaga has already matured into adulthood.

Luis Sera, a researcher who betrayed Saddler, built an operating machine - the Plaga Removal Laser - which produced this kind of radiation after extensive research regarding how the Plagas might be removed. Leon and Ashley, who had Plagas implanted into their bodies, used the machine to eliminate the Plagas, both managing to survive the operation.

Las Plagas are also very vulnerable to bright lights. They will only emerge from their hosts at night, and can be instantly killed by throwing a flashbang grenade at them.