Las Plagas are a breed of parasitic creatures that have appeared in Resident Evil 4 and Resident Evil 5. Their name is derived from plaga, the Spanish word for plague. Originally, these organisms were discovered centuries ago in rural Spain by a religious society known as Los Illuminados ("The Enlightened Ones"), who used the parasites to gain new members. The Salazars - a regal family - sealed the Plaga away underground after they witnessed the parasites' effects on people.
Resident Evil 4
In modern times, the Plaga parasites were unearthed again by Ramon Salazar, eighth castellan of the Salazar Family, who had been swayed by the then-leader of the Illuminados cult, Osmund Saddler. The parasites appeared to have died off during their isolation, but had survived at the microscopic level as spores. When the fossilized Plagas were excavated, the miners inhaled the spores which would develop into complete Plagas over a number of years, resurrecting the species. Lord Saddler used the parasites to take control of the local inhabitants of the region.
Saddler - a megalomaniac who harboured a deep resentment for the United States - wanted to use Las Plagas as his instrument for seizing control of the entire world. To this end, he and his followers hired genetic researchers and invested in medical technology to create a variety of bio-organic weapons that utilized the parasites. He arranged for the abduction of the US President's daughter, intending to infect her with a Plaga then send her home as a puppet under his control, who would infect the President with a parasite and bring the US government under the influence of the Illuminados. Saddler's plan was thwarted by US secret service agent Leon S. Kennedy, who fought the maniacal cultist and destroyed his organization. This did not spell the end for Las Plagas, however, as Ada Wong - an agent from an unnamed organization - had procured samples of both subordinate and dominant-strain Plaga, providing other parties the means to genetically breed the parasites.
Resident Evil 5
Las Plagas would reappear in 2009 in the Kijuju Autonomous Zone in Africa. Albert Wesker - a former researcher of the now defunct Umbrella Corporation and bioterrorist - had acquired the original parasite from Spain and was working with pharmaceutical giant TRICELL to enhance its properties and use it to produce a new breed of B.O.W.'s. Wesker had hired bio-weapons smuggler Ricardo Irving to unleash the parasites upon Kijuju, which was already in a state of civil unrest. Those infected with the parasites became highly aggressive and underwent bizarre mutations, gaining enhanced strength and resilience and in many cases turning into creatures that barely resembled a human being. With a newly designed Plaga strain, Wesker had created an army of superhuman puppets to assist him with his Uroboros project.
Subordinate-type Plagas are typically injected into a victim's bloodstream while still in egg form. When they hatch, the parasite quickly grows and will seize control of the host's higher reasoning functions, elevating a person's aggressive tendencies and enhancing the host's physical strength and endurance. Subordinate Plaga hosts can be manipulated by those who bare dominant-strain Plagas.
This Plaga breed was created via genetic modification and is capable of granting extraordinary abilities to a host without the host losing any of its mental faculties. Dominant-type hosts are capable of controlling subordinate-type hosts completely, compelling them to obey any command. They also completely retain their own will and the dominant-type Plaga can also trigger various mutations in its host. These mutations differ with each host.
Leech Plaga have only been found infesting the bodies of Regeneradores, humanoid B.O.W.'s developed by Los illuminados. Several of these parasites inhabit a single Regenerador and contribute to its ability to rapidly regenerate any wounds it receives. The only way to quickly kill a Regenerador is by killing the parasites within the body, which can only be seen using thermal imaging equipment.
This Plaga variant was created by TRICELL after researching the original Plaga samples recovered by Wesker. By combining the original Plaga with leech DNA, TRICELL developed a parasitic organism capable of maturing without a host that could then be implanted into a person and take control of them in a matter of seconds as opposed to hours or days with the original Plagas. While the original parasites could only take full control if the host was willing, Type-2 Plagas could seize control of unwilling victims as well. TRICELL infected many residents of Kijuju by force-feeding the parasites to a handful of people and then commanding them to infect others.
Similar to the Type-2 but created by introducing the genes of a dominant-type Plaga to a subordinate-type. Like the Type-2, it could be force-fed to individuals and turn them into mindless puppets within seconds, though this type could trigger more visible mutations in its hosts. TRICELL infected the Ndipaya - the native aborigine tribe of Kijuju - with Type-3 Plagas and many of them displayed visible changes such as expanded muscle tissue in the forearms and expansion of the heart causing it to erupt from the person's chest. However, this exposed organ did not represent a weakness. The infected Ndipaya displayed a great increase in strength, speed and dexterity, and even seemed to be capable of making tremendous jumps. Some of them even increased in height, becoming as tall as 3 meters.
All Plaga variants are vulnerable to strong light. Ultraviolet rays are extremely harmful to them and using weapons such as flashbang grenades against an exposed Plaga will instantly kill it and cause it to disintegrate.