The Las Plagas parasite
The Las Plagas parasite has the ability to manipulate and change the behaviour of it's host. Seen in Resident Evil 4 the Las Plagas parasite brought Resident Evil to an entirely different level. Unlike the T-Virus it allows the infected (host) to retain intelligence, thus, infected humans - Los Ganados are capable of wielding weapons and following complex orders, often with ruthless efficiency.
 Spread of the Plaga
The Las Plagas parasite has proven itself to be incredibly efficient. Observed by Luis Sera, the parasite works it's way up the food chain and takes control of more advanced organisms e.g. Birds, increasing the spread radius of the parasite and thus it's success:
"Diciocoelium - Once the larvae of this parasite migrates to the ant's esophagus, it alters the behavior of the ant. When the temperature drops in the evening, the infected ant climbs to the top of a plant and clamps onto a leaf using its mandible. It stays there immobile until the next morning, placing the ant where it's most vulnerable to be eaten by a browsing herbivore such as sheep. One could conclude that the parasite is manipulating the host's behavior to make its way into the body of its definitive host.
Galactosomum - The larvae of this parasite makes its home inside the brain of a fish such as the yellowtail and the parrot bass. Once infected, the fish make their way up to the water's surface, where they'll swim until being eaten by seabirds. Once again, this peculiar behavior can only be explained by the parasite's desire to get into the bodies of the seabirds.
Leucochloridium - This parasite's sporocysts develop in the snail's tentacles. The sporocysts are vivid in color and pulsates continuously somewhat like a worm. Surprisingly, the infected snail makes its way to the top of a plant where it is more visible to the eyes of birds. Therefore more likely to be eaten. Once eaten by a bird, the parasite will complete its metamorphosis into an adult."
"For many years the Salazar family has served as the castellans of this castle. However, not everything is bright, for my ancestry has a dark past. Long ago there once was a religious group that had deep roots in this region called the Los Illuminados. Unjustly however, the first castellan of the castle took away their rights and powers. As a follower of this religion and as the 8th Castellan, I felt that it was my duty as well as my responsibility to atone for that sin. I knew the best way to atone for that sin was to give power back to those who we once took it away from, the Los Illuminados. As expected it took a little time, but we were able to rejuvenate the once sealed Las Plagas."
The Los Illuminados group had once used the Las Plagas parasite as a tool, a tool to increase their numbers and spread their 'religion', the first Castellan saw through the religious front and took action - the Plagas were buried underground in an attempt to erase them from existence. Many years later, under the illusion that Lord Saddler had good intentions, Salazar, the now 8th Castellan decided to excavate the Las Plagas. In doing so, he inadvertently gave them new life. Years later the villagers who had taken part in the excavation of the Plagas started convulsing and showing signs of apparent illness, shortly after the same villagers had turned into crazed lunatics. The Las Plagas had survived - in spore form, a testament to their durability and efficiency.
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