Fictional viruses in the Resident Evil series

One of the major plot elements of the Resident Evil series is the outbreak of various viruses. Typically, these viruses are the creation of the fictional Umbrella Corporation, which is attempting to use them to create BOWs (genetically engineered soldiers), and the results of a leak have lead to mass zombification of people and are the premise of most games.

Origin
According to the series timeline, after the Ebola virus was discovered in Africa, Umbrella decided that it would make an efficient bio-weapon due to its near 90% death rate. It obtained samples of the virus, circumventing the fictional Biological Weapons Prohibition Pact by claiming that they were researching it in order to find a cure. However, the virus proved unsuitable for their uses, as it cannot survive for long outside of a human host, and is extremely vulnerable to UV radiation. Furthermore, it kills its host rapidly, and is only contagious through direct contact, severely limiting its ability to spread. Umbrella decided to create a new virus based around the idea of a heavily infected host that could still move around and actively infect others.

Progenitor virus
The Progenitor virus, also known as the Founder or Mother virus, is the first of the mutagenic viruses, and the basis for most of them.

According to the series timeline, it was discovered on December 4, 1967, Dr. Edward Ashford, Dr. James Marcus, and Lord Ozwell E. Spencer, while they were studying the Ebola virus. Though Ashford wanted to use the virus' regenerative abilities to help the handicapped, Marcus and Spencer wanted to use it for the bio-weapons project, and after Ashford's death in 1968, they were able to begin their desired research.

It produces rapid and uncontrollable mutation in a host's genetic code, but the mutations were not coordinated enough to produce effective BOWs. In hosts with a genetic structure less complex than humans, mutations are less pronounced, and usually restricted to increase in size and aggressiveness. In order to enhance the virus's mutagenic properties, Umbrella created a variant of it through synthesizing it with the Ebola virus, but this strain retained Ebola's photosensitivity.

The Progenitor virus was originally going to be used in Resident Evil 4 in the fog version of the game, but was instead replaced by the Las Plagas.

T-Virus
The Tyrant virus, or T-Virus for short, is the main virus used by Umbrella in the series, and is responsible for the creation of most their BOWs.

According to reports in the game, Dr. Marcus, Albert Wesker and Dr. William Birkin succeeded in creating the Tyrant virus at the Arklay Labs on December 4, 1978 through synthesis of the Progenitor-Ebola virus and leech DNA. Through this synthesis, the photosensitivity of the early Progenitor-Ebola strain was replaced with pyrosensitivity, a property that can be seen in most of the series's enemies.

The Tyrant virus supposedly operates similar to most other viruses, but also has the abilities to animate dead tissue, to substantially mutate its host, and to infect nearly any tissue in any type of host. It animates dead tissue by killing and replacing any mitochondria in infected cells, and then combining with these cells to produce enough energy for motor and lower brain functions. By doing this, most of the bodies systems, such as the circulatory or respiratory systems, are made redundant. However, this process has the drawback of severe necrosis in the host, and produces the distinctive rotted appearance of most BOWs. The mutations are produced when the virus incorporates itself into the host's RNA and considerably alters it. Creatures with genetic structures less complex than humans generally show less severe mutations, and usually only increase in size.

As a side effect of the virus's cannibalization of its host, specifically its digestion of the host's frontal lobes, all hosts suffer from greatly increased aggressiveness. In previously living hosts, the virus also damages the hypothalamus, which results in a flood of neurotransmitters, enzymes, and hormones which induce a psychotic rage and hunger in the host.

T-Veronica virus
The Tyrant-Veronica virus is the main virus used in Resident Evil: Code Veronica, and is responsible for the mutation of several of the enemies in the game. There is also a variant strain of the virus named T-Alexia that is a diluted form of the virus created during its period of cryogenesis in its main host, Alexia Ashford, who is also the main antagonist of the game.

According to the series timeline, the T-Veronica virus was created by Alexia Ashford through synthesis of the Progenitor virus and the gene of a queen ant that contained an "ancient virus". She administered it to her father, but because the infection was not controlled, he became unstable and mindless. This led her to discovery that the mutations could be controlled if slowly absorbed during cryogenesis. It is also worth noting that no in-game B.O.W infected with the T-Veronica virus shared the pyrosensitivity of the T-Virus; whether it shares the photosensitivity of the Progenitor virus is never shown.

Like the Progenitor virus, the T-Veronica virus causes rapid mutation, but the infection can be controlled and intelligence can be retained if the virus is absorbed over a long period of time — Alexia Ashford achieved this by remaining in a cryogenic sleep for fifteen years while the virus merged with her at a cellular level. Among the mutations to Alexia caused by the virus are the ability to exert control over the hive mind of the ants the virus was extracted from, combustible blood and vomit, and multiple tentacle-like muscles. It is suggested within the game that Alexia can control her mutations, though they become more severe after she sustains considerable damage in the player's final boss fight with her.

Though the T-Veronica virus is a useful tool for transhumanism purposes, it is useless as a bio-weapon, as its infection rate is low and it can only be transferred through direct injection.

NE-T virus
The NE-T virus is very similar to the T-Virus, but was designed specifically for the Tyrant Project.

According to the series timeline, Umbrella required BOWs that were not only stronger, but could understand and complete missions, and due to the nature of the T-Virus, all of its BOWs were unable to do this. The project was divided between an American team, led by Dr. Birkin, which would work on increasing the BOWs' strength, and a European team which would work on increasing their intelligence. The two teams traded theories and ideas that included the first experiments in directly operating on the subject's brain. The American team succeeded in creating the NE-T virus, along with the T-002, which could follow simple orders such as "Restrain" and "Attack", though its intelligence was insufficient for field operations.

In response to reports of the T-002’s failure, Umbrella renamed the European team's project with the title "Goddess of Vengeance", or "Nemesis" for short, and gave it the new goal of eliminating all anti-Umbrella elements (specifically, the STARS members). The European team made a breakthrough with the creation of the NE-α, an organism created from the NE-T virus that affected the host's brain. Further experiments with this strain led to the creation of the T-103, also known as "Mr. X".

G-Virus
The 'Gene virus, or G-Virus for short, is the driving factor of the events of Resident Evil 2. The only known successful host of the virus is the character Dr. William Birkin, who dies at the end of the game.

According to the series timeline, it was developed by Dr. William Birkin after its basis was discovered in the mutated body of Lisa Trevor. She had displayed an unexpected immunity to the NE-α parasite that they tested on her, and Birkin and his associates found what would become the G-Virus as the cause of this immunity. It turned out that the G-Virus was a mutation of the Progenitor virus she had originally been infected that arose by absorbing all the other mutagens she had been implanted with. Dr. Birkin received approval from Umbrella to begin studying the virus, and completed his work on it in 1998. However, due to disagreements with his superiors, he decided to keep the research to himself and made a deal with the U.S. military to extract him. In response, Umbrella sent their Special Forces Unit to steal his research and virus samples. The unit shot Birkin and left him for dead, but while dying, he injected himself with a sample of the virus, and became the creature known as G. He tracked the USFU team to the sewers, and during the ensuing onslaught, several T-Virus vials were dropped into the water, inciting the city-wide infection of the game.

The mutations induced by the G-Virus tend to be extremely volatile. Unlike the T-Virus, the mutations caused by the G-Virus occur exponentially faster, and continue to be induced so long as the host is active, even lacking external stimuli, though they are more rapid and substantial when the host is wounded. Also, the G-Virus possesses none of the necrotizing properties that the T-Virus is known for. Furthermore, the virus is transmitted through implantation or ingestion, rather than physical contact or injury. It can only be accepted fully by hosts with a suitable genetic makeup, specifically a blood relative, so during the game, G seeks his daughter Sherry Birkin in order to successfully reproduce. However, it can also partially infect others by implantation of G-embryos, producing severe mutations that culminate in the host being able to release further G-embryos. Hosts, called G-mutants in the game, create their offspring by orally implanting small, parasitic organisms into a living host through the palm of their hand. If the host is compatible, the embryo begin to pupate and assimilate the host's body, turning them into a mutant of immense strength. If the host is incompatible, the embryo will rapidly grow inside the host's body, and will burst from their chest in a matter of minutes. They will then quickly mutate into their adult form and search for another host.

Though the G-Virus is a somewhat useful tool for trans-humanism purposes, it is inefficient as a bio-weapon, due to its poor communicability. However, it is more aggressively contagious compared to the T-Virus, which does not actively seek to infect others.

The G-Virus can be cured by DEVIL, an experimental vaccine. However, the vaccine is only effective during the initial stages of an impregnation-induced infection, before the embryos gestate.

Wesker's virus
This unnamed virus only appears in the original Resident Evil, and was removed from the remake released in 2002. As such, it is unclear if it remains canon to the series' storyline.

According to the original Resident Evil, this virus was created by Dr. William Birkin for Cpt. Albert Wesker to use when he faked his death in 1998.

The virus hastens the host's metabolism, giving them heightened strength, speed, and regenerative abilities. Unlike the other viruses featured in the series, it is not a severely mutagenic virus, and its effects are limited to muscle tissue and lower organs, leaving the nervous system untouched. Though Wesker is responsible for an impressive list of horrific crimes, it is unknown if the virus was responsible for his psychotic behavior, or if that was his natural disposition.

As a prerequisite to increasing the host's speed, the virus also affects ocular tissue, allowing it to view and process visual information at a speed allegedly equatable to raptors. This modification of the eyes is the only visible effect of the virus on its host.

T/G Virus
The T-Virus is the basis for Resident Evil: Dead Aim 's T/G Virus. It is, as it's name suggests, a fusion of the T- and G-Viruses. It imbues it's carriers with qualities of both it's parents strains, as well as odd electrical properties.

G-Virus G-Vírus