The world of Resident Evil's biological monstrosities and the viruses, fungus and parasites that create them is no simple task to digest, especially when considering the vast history of Resident Evil's viruses. Understanding these, however, is key to fully grasping what transpires in the later games much easier and more clear.
Following the establishment of the T-Virus' origins and the downfall of Umbrella, the events which transpire after become much more complicated in the Resident Evil universe as other companies and black market profiteers pick up where Umbrella left off, and plenty of new monstrosities are introduced.
Las Plagas parasites and Tricell (Resident Evil 4 and 5)
The Las Plagas, or just Plagas, are the parasites that take over the hosts, as seen in Resident Evil 4 and 5. These parasites have their own history and origins, and they are not connected in any way to Umbrella or Resident Evil's previous infections, viruses, or parasites.
Specifically, these Plagas parasites are anthropods, and have lived for a long time in an isolated and mountainous enclave somewhere in Spain. In the 18th and early 19th centuries, a leader named Salazar led an invasion of the region against religious zealots who had utilized the powers of the Las Plagas in some way or another. Salazar defeated this group, known as Los Lluminados, and built a fortress above the underground cave complex of the Plagas' habitat, promising to keep the parasites trapped in there, so they would no longer pose a danger to the outside world.
Jump ahead, and Osmund Saddler, a bio-weapons researcher with a mysterious origin, seemingly working for his own interests and no one else, came to the area and was able to strike a deal with Ramon Salazar, the protector of the area and descendant of the original Salazar. Saddler obtained access to the Plagas in the caves and implanted them into nearby villagers to use as hosts, who became possessed under his orders, as he presented himself as the Prophet of the revived Los Illuminados cult.
On a nearby island, Los Iluminados began work on genetically-engineering the parasite, creating three new species from them. The first were the Recessive Plagas, which controlled the majority of the villagers in the game. The second was the Dominant Plagas, which Saddler, Salazar and the village priest, Bitores Mendez, used to infect themselves with, in order to lead those infected with the Recessive Plagas. Finally, the third was the Queen Plaga, which lived in the castle and asexually created eggs to produce more Plagas parasites.
Los Illuminados ultimately wished to infect U.S. government officials with Recessive Plagas, including the President, in order to recruit them as agents to gain power and influence over the country. Meanwhile, Ada Wong was dispatched by the mysterious employer, known simply as "The Organization," to obtain a Dominant Plagas sample. At the same time, Albert Wesker was also trying to obtain a sample as well.
By the end of Resident Evil 4, both Ada and Wesker both obtained their own samples of the Dominant Plaga. Albert turned his loyalties to a company called Tricell, while Ada remained seemingly loyal to "The Organization", whatever that entity may be, as it still has yet to be revealed in Resident Evil lore.
Tricell was part of a larger conglomerate of companies known as the Federation of Pharmaceutical Companies (FPC), which Umbrella was also a member of, before its downfall.
Interestingly, Tricell pre-dates Umbrella going back to the 18th century as an international trading company known as Travis Enterprises. Tricell founders' descendant, Henry Travis, made expeditions into West Africa in the 19th century, noting a brief discovery of a mysterious flower that would later give birth to the infamous T-Virus. Umbrella's founders then went searching for Henry Travis' rumored flower.
Tricell missed out on the whole T-Virus escapade that engulfed the games of Resident Evil 0-3 and Code Veronica, and the famous Raccoon City incident, despite having a role in the original discovery of the flower that started the whole mess. But Tricell conveniently had its hands on a Dominant Plaga sample, thanks to Wesker's services, and "The Organization" also had its own sample, thanks to Ada. With this, Tricell made the more powerful Type 2 and Type 3 Plagas seen in Resident Evil 5 in West Africa.
T-Virus and G-Virus Fall Into New Hands
Meanwhile, WilPharma Corporation was originally responsible for making the T-Virus Vaccine. Much public controversy transpired when leaks exposed its clinical tests on human test subjects in India, who were infected with the T-Virus in order to test the vaccine. This led to the company's collapse.
But many wondered how WilPharma had T-Virus samples to begin with. As it is discovered, one of WilPharma's employees, Frederic Downing, was an Umbrella employee long ago. As it turns out, he was able to obtain both T-Virus and G-Virus samples from Umbrella's facility under Raccoon City around the events of Resident Evil 2 and 3.
Downing likely obtained the samples soon after Dr. Birkin was killed and unleashed the T-Virus in the sewers as chaos slowly began to simmer, but did so before full blown lawlessness ensued, as he was not a visible character in either game and was long gone by the time players made it to the underground facility. After WilPharma's collapse, Tricell purchased the company's assets, including its abandoned Air Dome Laboratory, where Tricell obtained leftover samples of the G-Virus.
It was also discovered that Downing made secret deals with bioterrorists on the black market, selling them T-Virus samples, hoping that the vaccine would make solid profits if outbreaks occurred around the world.
Uroboros Virus and Tricell's End (Resident Evil 5)
In exchange for Albert Wesker's contributions and favors for Tricell, he was able to get the company to support and back work development of the Uroburus virus, potentially the strongest, most chaotic, and seemingly destructive virus in The Resident Evil universe. To create such a virus, it required an unaltered original strain of the Progenitor virus, which was derived from the flower that Umbrella's founders originally obtained in West Africa.
To accomplish this, Tricell was able to locate Umbrella's old research center in Africa, yet the first Uroboros virus sample was too strong, killing anyone it was tested on. Jill Valentine proved to be the perfect solution, and she was captured. Fans can recall her role in Resident Evil 3 as she was cured of the T-Virus, using methods that were different and were made long before WilPharma's T-Virus Vaccine.
As a result, this left special T-Virus antibodies in her body, and utilizing samples of these antibodies helped develop a more stable Uroboros virus which could transform successful hosts with greater odds, giving them superhuman abilities. Its intention was to be controllable enough that it would leave a certain percentile of human populations alive.
Albert Wesker had planned on unleashing this virus into the atmosphere of Earth and to infect all of the world. Soon after, Tricell was shut down as it was revealed that its CEO was responsible for trying to kill a BSAA unit that posed a threat to the Uruboros Plan, which was essentially the storyline of Resident Evil 5.
Mold (Resident Evil 7)
The Mold, responsible for the enemies seen in Resident Evil 7, was created by The Connections in the 1990s in order to utilize mind control in war, and is a fungal super-organism. The Connections represents a crime syndicate, which had hopes to sell child subjects as Bio-Weapons to wealthy buyers in order to orchestrate complex assaults against enemy networks. This is done by using mind control schemes and low-profile infection that would go unnoticed by most observing eyes, such as local governments or law enforcement in areas.
The Mold was finalized around 2000, with help from Albert Wesker's private unit of armed forces specialists. It is unclear exactly what the process was, and whether or not it had any connections to previous research, such as Umbrella's T-Virus.
The Mold is able able to transform enemies into allies, then read their minds to obtain new information, while utilizing the hosts to gain more recruits to infect. This is how The Mold, through Eveline, was able to control Mia and have her record a video message of herself, send it to her husband, Ethan, and ultimately convince him to travel to her location in order to try to infect and control him as well.
Still More Unanswered Questions
There are still many questions abound, such as the identity of "The Organization" that Ada Wong worked for, what happened to all the samples Tricell had after it shutdown, whether the Las Plagas viruses will eventually make a return and find common-ground with the T-Virus lineage of viruses, the status of the Dominant Plagas sample Ada gave to "The Organization", which entities on the black market now have viral samples, and much more.
Hopefully, Resident Evil 8 and/or the remake of Resident Evil 3 will give us deeper insight not just into Umbrella, but also clarify the competing companies and relationships with various black market entities in the Resident Evil universe.