quote Resident Evil Wiki

Los Illuminados (From Spanish meaning "The Enlightened Ones") was a cult operating in an unknown rural region in Spain.

It was created some time around the 1700s where they worshipped "Las Plagas" (Spanish: "The Plagues"), a parasitic organism with the ability to control its hosts.

The 1st Castellan of the region outlawed the cult and had the Plagas sealed in a cave system beneath his castle. Eventually under the rule of the 8th Castellan, the Los Illuminados now under the rule of Osmund Saddler, were allowed to mine the area, for that was how salazar felt he should atone for the sins of his ancestors, who sealed Las Plagas from the Los Illuminados,[1] effectively occupying his castle which acted as a base for the cult under Saddler.

When Ramon Salazar released the Plagas seal no one thought they could bring them back to life. However Plaga remains were found to be fossilized. (suggesting they could be millions of years old) Although they appeared fossilized, they were able to survive the long years by lying in a dormant state at the cellular level remaining in a spore-like form. These were found to be what first infected the miners, becoming full fledged Plagas after many years of development. Likely using these first Plagas, Saddler perfected the ability to reproduce the organism and took to implanting Plagas eggs into new hosts.

The entire Los Illuminados were soon implanted with Plagas along with much of the local population who had joined the cult. Saddler himself was implanted with a Master Plaga and as such has direct control over the others using lieutenants with similarly augmented Plagas to monitor and direct the Ganados on the ground. Any who resisted the Plagas were executed or used as research in the cult's genetic manipulation experiments.

The villagers then returned to life as normal as they could, farming, attending cult services at the local church, acting as guards for Salazar's castle or working the mines while the cult zealots engaged in dark rituals and sacrifices in the name of Saddler.

Eventually, American agent Jack Krauser, while secretly working for Albert Wesker arrived at the village in search of the parasite. He was later used to kidnap Ashley Graham, daughter of US President Graham as part of an Illuminados plot to do no less than take over the entire world by infecting Graham and returning her to her father in order to infect the United States executive. Plan B was to use Saddler's paramilitary militia to infiltrate the United States and sow discord from within, likely gaining converts to Los Illuminados as they went.

Los Illuminados is believed to be destroyed with the death of Saddler and the Master Plaga.

quote Resident Evil Wiki

It is unclear if this level of assimilation would eventually be demonstrated by all those infected by Las Plagas, or if Mendez was subject of experimentation like many of the game's monsters.