T-Virus

The Tyrant Virus - more commonly referred to as the T-Virus - is a man-made mutagenic plague and a staple of the early Resident Evil games. Created by the Umbrella Corporation, the T-virus was derived from Progenitor, a non-carcinogenic virus originally discovered in West Africa. Umbrella and other organisations have used the T-Virus as a catalyst for producing various types of biological weapons.

Development
In the early years of the Umbrella Corporation, the company's three founders - Oswell E. Spencer, Edward Ashford and James Marcus - attempted to extract the Progenitor Virus from its natural African habitat and conduct research upon it in facilities within the United States. This proved to be difficult as the virus could not survive outside its natural environment, but eventually Umbrella built the necessary facilities to recreate the proper conditions to house the Progenitor flowers and extract the virus.

Throughout the 1970s, Marcus carried out extensive research upon Progenitor, experimenting with its effects on different species in order to produce an efficient bio-weapon. Many tests failed, but Marcus later came upon the idea of combining the virus with leech DNA. The attributes of Progenitor and the leeches meshed well together and by 1978, Marcus had created the first primitive strain of the Tyrant Virus.

Though most of the research for the virus took place within the Arklay Laboratory underneath the Spencer Mansion facade, some eight miles northwest of Raccoon City, and the Umbrella training facility under Marcus' orders, covered by the American Umbrella offices, Spencer's Umbrella empire in Europe raced against Marcus to adequate the T-Virus to create a virus with even greater destructive potential. Marcus' dreams to overthrow Spencer and his attitude towards the virus as a "money-spinning tool" for Umbrella were never fulfilled, as he was murdered before he could enact them. William Birkin came to be known within Umbrella circles as the true creator of the T-Virus.

Research on the T-Virus continued for many years following Marcus' death and saw the development of several T-viral offshoots, created not only by Umbrella but other illicit groups as the virus was later distributed over the global black market.

Effects
The T-Virus is built like many viruses. As the virus makes contact with a cell's membrane, it inserts its genetic coding into the cell. The cell absorbs the viral genome into its own genetic make-up, which takes over the cell's functions. The cell begins to produce offspring of the original virus. The new viruses are then released from the host cell and infect the neighbouring cells, which starts the process all over again.

The T-Virus can only properly function by consuming the cell in order to produce its energy. This slow breakdown of cells leads to the gradual breakdown of the host, causing necrosis. The virus incorporates itself into the host's DNA, which substantially alters it. This is why creatures, such as bees and spiders, only have very minor mutation, such as increased size, when compared to the human mutation caused by the virus.

Should the human host be alive at the time of infection, all higher brain functions are destroyed. This leaves only the cerebellum to govern behaviour. This leaves the infected host with a very animalistic behaviour. As the virus spreads, it damages the hypothalamus. This produces a massive flood of neurotransmitters, enzymes, and hormones such as norepinephrine and dopamine. These effects, combined with the painful symptoms of the infection, induce a psychotic rage, persistent hunger, and increased aggressiveness in those infected.

Should the human host be dead at the time of infection, only enough energy is produced to power the lower brain functions. These functions are responsible for the most primal instincts, such as walking and eating.

In either case, their late-stage appearance and behaviour are characteristics of their name-sake, a zombie.

Should a late-stage human host be rendered unconscious, the body will lay in a dormant phase appearing to be dead. In actuality the host is rapidly mutating due to the T-Virus becoming hyperactive. Within the time frame of an hour, the decayed skeletal and muscular system is reconstructed into a stronger and more durable form. Development can be seen of sharp canine teeth and razor claws, fresh blood seeps into remaining skin giving it a reddish hue, advances in decomposition, and intact eyes become a bright white. Umbrella researchers at the Arklay Laboratory have dubbed this stage "V-ACT", referring to the resulting creature also known as a "Crimson Head". To awaken, the brain must be jolted back to consciousness with a slight but important increase in intelligence, and large increase in agility and aggressiveness. As a consequence of this rapid mutation, cellular breakdown hits a critical point where the host cannot regenerate properly and can be killed permanently.

Depending upon the strain, the V-ACT may have different results. In the strain released in Raccoon City, most V-ACTs usually resulted in creatures named by RPD staff as "Lickers" which are completely blind and use their hearing to detect prey. At which point, they use their long tongues to strangle, impale, or decapitate their prey. At close quarters they also have sharp claws to slash their prey. Lickers may eventually evolve into a more advanced stage with larger claws. Lickers also have exposed brains which serve as a weak point.

An oddity of V-ACT is a creature known as "Suspended". Only one specimen of this creature is known to exist. What is actually is, is unknown. It could be an in-between stage of a zombie and Licker or some kind of rare mutation V-ACT takes on. If it is a zombie partially evolved into a Licker, it would account for why its unable to spear prey with its tongue. It's only capable of grabbing its prey and using its tongue as a whip.

Albert Wesker notes that he observes similarities in the apparently random mutations endowed by the virus (as examples he mentions the mutations undergone by Sergei Vladimir and what he knew of James Marcus' mutations) and explains he believes that the host's mindset is a crucial factor in explaining the mutation (e.g.; Vladimir becomes what Wesker calls "a monster whose very body is the picture of suffering", revealing what Wesker considered to be Vladimir's true self, and the rejuvenation experienced by Marcus, an aged man who understandably desired control and the restoration of his vibrant youth, characteristics that were carried to his own eventual mutation, including his true madness and the horrific mutation that overcame him in the end), and briefly wonders where does this theory fit with his own mutations.

Biohazards
There have been multiple occurrences of the T-virus being released within populated areas, infecting humans and other forms of life and resulting in widespread destruction. A list of such disasters has been compiled below.
 * The Mansion Incident - In the summer of 1998, a clone of James Marcus formed by the fusion of the deceased Marcus' DNA and T-infected leeches appeared in the Arklay Mountains, scattered the T-Virus and infected both the passengers of the Ecliptic Express train and the staff and residents of the Spencer Mansion. As the zombified staff and B.O.W. specimens ran wild, a number of horrific murders occurred in close proximity to Raccoon City. The Raccoon Police Department dispatched its elite S.T.A.R.S. unit to investigate; they discovered, at great cost, that Umbrella were responsible for developing biological weaponry. Following the destruction of the Arklay Lab and Training Facility, the S.T.A.R.S. attempted to blow the whistle on Umbrella, but the corporation used their vast wealth and political pull to ensure that the survivors' tales of monsters and conspiracies went unheeded.
 * The Raccoon City Destruction Incident - Six weeks following the Mansion Incident, the T-Virus began to spread through Raccoon City due to a combination of contaminated run-off from the mansion leaking into the city's water supply and the release of the virus in the city sewers after a botched operation by Umbrella Special Services to retrieve the G-Virus from William Birkin. Within two weeks, virtually the entire population of Raccoon was either dead or infected with only a handful of survivors managing to make out of the city alive. In order to prevent the virus from spreading further, the US government was forced to launch a thermobaric non-nuclear missile to destroy Raccoon City, wiping it off the face of the Earth. Umbrella tried in vain to cover up the disaster but were soon exposed for their development of the T-Virus. While they did manage to drag out the legal proceedings for several years, the fall of Raccoon City spelled the fall of Umbrella. The company was found guilty in 2003 and prominently shut down.
 * The Sheena Island Outbreak - One month following the Raccoon City Incident, the Umbrella B.O.W. plant on Sheena Island was destroyed when the T-Virus was purposefully released by the facility's director, Vincent Goldman, who had attempted to make the outbreak appear accidental. His plan was to stabilize the situation himself and route out traitors in the company in order to advance his own position and reap massive profits as a result. His plan failed largely due to Ark Thompson and Goldman himself was killed by the Hypnos T-Type.
 * Rockfort Island and Antarctic Outbreaks - In December 1998, the Umbrella facility on Rockfort Island was attacked by a private militia force led by former Umbrella operative Albert Wesker. The T-Virus was released from the island's lab complex, infecting most of the staff and prisoners on the island. The reason behind the attack was so that Wesker could acquire the T-Veronica virus which was believed to be on the island, but in truth, the Veronica strain was contained at the Umbrella Antarctic Base. Claire Redfield and Steve Burnside - two escapees from the Rockfort prison complex - managed to escape the island before its self-destruct system was triggered. However, their escape plane was remotely diverted to the Antarctic by Rockfort's director Alfred Ashford, as was another plane carrying a handful of surviving island staff. The escaping staff had actually contracted the the T-Virus and succumbed to the infection by the time they arrived at the Antarctic Base, thus spreading the virus there as well. It was at the Antarctic facility that Claire was reunited with her brother - ex-S.T.A.R.S. member Chris Redfield - and together they destroyed the mutated Alexia Ashford and escaped before the base self-destructed.
 * The Spencer Rain Incident - In 2002, former Umbrella researcher Morpheus D. Duvall hijacked an Umbrella cargo freighter, the Spencer Rain. He and his group of followers threatened to attack both the United States and China using the T-Virus and Morpheus even went so far as to contaminate the entire ship with the virus, mutating all of his supporters. US STRATCOM agent Bruce McGivern was able to stop Morpheus from carrying out his scheme and destroyed the ship.
 * The South American Outbreak - In 2002, US agent and Raccoon City survivor Leon S. Kennedy and fellow agent Jack Krauser were dispatched to South America to arrest notorious drug-lord Javier Hidalgo, who was suspected to have ties to Umbrella. Hidalgo had acquired the T-Virus and several B.O.W.s off the black market in order to secure his private holdings and protect him whilst he experimented with the T-Veronica virus. He infected his mercenary group with the T-Virus and unleashed them upon Leon and Krauser, but the two agents were able to defeat every creature that Javier sent their way. Javier then infected himself with the Veronica strain and transformed into a huge monster, but was also defeated.
 * The Caucasus Raid - In 2003, in Umbrella's final days, a group of anti-Umbrella privateers led by Chris Redfield and Jill Valentine launched an assault on Umbrella's last remaining B.O.W. research facility in Russia. In order to prevent them from interfering with the T-A.L.O.S. project, Sergei Vladimir released the T-Virus inside the facility, turning his staff into zombies, and activated every B.O.W. in the complex. After a long, gruelling battle, Chris and Jill finally made it to the centre of the facility and destroyed T-A.L.O.S.. Sergei himself was killed in battle against Albert Wesker who had infiltrated the facility in order to acquire Umbrella's complete database from the U.M.F.-013 supercomputer, furthering his own plans. Umbrella collapsed completely following this event.
 * The Harvardville Airport Outbreak - In 2005, an unknown bioterrorist group launched a two-pronged biological attack upon the American town of Harvardville as part of a twisted sales pitch orchestrated by WilPharma researcher Frederic Downing, who was selling the T-Virus to terrorists and dictators across the world. The virus was released aboard a commercial airliner bound for Harvardville and in the airport terminal itself. The plane crashed into the terminal, unloading its zombified passengers and adding to the number of zombies already present. A Special Response Team led by Leon S. Kennedy was dispatched to rescue any survivors of the disaster and to eliminate the infected presence. The operation was successful and the outbreak was contained and destroyed before it could spread beyond the airport.

Mutations
Below is a list of different creatures that have been created via the T-Virus, whether through accidental exposure or purposeful infection as part of bio-weapons development.

Irregular mutants

 * Zombies - Human beings infected with the T-Virus are transformed into creatures resembling the living dead of horror fiction, mindlessly shambling about even as they continue to rot, seeking to consume the flesh of the living.
 * Zombie dogs - Canines infected with the T-Virus that undergo similar mutations to human victims, partially decomposing and becoming extremely violent. Umbrella has also intentionally produced T-infected dogs (usually Dobermans) as B.O.W.s, dubbing them as "Cerberi".
 * Crimson Heads - Zombies that have undergone a V-ACT mutation, becoming faster, stronger and more aggressive. They are so named by the reddish colouration of their skin.
 * Lickers - Another V-ACT mutation in human infectees, one which has also been utilized as a B.O.W.. These quadrupedal beasts use their clawed limbs for scaling walls and ceilings and can stab or decapitate prey with their tongues.
 * Suspended - A unique V-ACT that appears to be an intermediate stage between Crimson Head and Licker. This one-of-a-kind specimen appeared during the Raccoon Incident.
 * Crows - Infected birds only underwent a marginal mutation, slightly increasing in size and becoming more aggressive and territorial.
 * Bats - Infected bats do not undergo any obvious physical mutation, but they do become much more aggressive and their saliva can infect others. One specimen was reported to have grown to a gigantic size, large enough to lift an adult human off the ground.
 * Adders - These snakes do not undergo any visible alteration, but do become more aggressive and their venom becomes more potent.
 * Spiders - During the Raccoon Incident, several arachnid specimens succumbed to secondary infection, growing exponentially in size and aggression and developing extremely potent venom. Umbrella had also previously attempted to produce spider-based B.O.W.s called Web Spinners.
 * Leeches - The leeches produced by James Marcus were used for experimental purposes only and were not considered viable as weapons, though Marcus' clone put them to great use in contaminating Umbrella's training and research facilities in the Arklay region. Another group of infected leeches was also present in Raccoon City during the outbreak, though these were created by accidental exposure.
 * Plant 42 - A giant, mutated plant present in the Spencer Dormitory during the Mansion Incident.
 * Alligators - Two infected alligators were present in Raccoon City at the time of the outbreak; one was believed to be an exotic pet that escaped into the sewers while the other was an infected zoo animal. In the case of both specimens, they grew to gigantic proportions.
 * Cockroaches - These insects undergo a slight increase in size and act in a pack or hive mentality, grouping together to attack much larger prey.
 * Sliding Worms - Millipedes that have been transformed by the T-virus into vicious lamprey-like bloodsuckers.
 * Grave Digger - A giant adult version of the Sliding Worm. It creates tremors as it burrows through the ground and can swallow human prey whole.
 * Drain Deimos - Fleas that have succumbed to secondary infection. They have developed wickedly sharp hooks on the end of each limb for climbing surfaces and they feed on living prey by sucking out their cerebral-spinal fluids.
 * Brain Suckers - Similar to the Drain Deimos, but these creatures have two heads and feed on the brains of their victims.
 * Centurion - A giant centipede encountered in the Umbrella Training Facility. It is unknown if this creature was an early B.O.W. experiment or just an accidental mutation.
 * Lisa Trevor - A human guinea pig imprisoned by Umbrella at their Arklay laboratory and subjected to numerous viral experiments over a period of decades. Incredibly, she survived each of them and was transformed into a deranged creature that was near-impossible to kill. She was finally destroyed when the Arklay facility self-destructed.
 * Sergei Vladimir - Head of Umbrella's paramilitary forces and director of the Caucasus facility. When confronted by Albert Wesker in 2003, he injected himself with a T-virus strain that turned him into a grotesque monster. In this mutant state, he retained his intelligence and personality, not that it mattered as he was killed by Wesker shortly after.

Bio-Organic Weapons

 * Hunters - The first marketable B.O.W.s Umbrella produced; created by fertilizing a human egg with reptile DNA using the T-virus as a bonding agent. Multiple Hunter types have been created and have remained available on the black market since Umbrella's downfall.
 * Tyrants - Human-based B.O.W.s created through T-infusion and advanced surgical procedures. Only a small percentage of human subjects possess the necessary genes for the Tyrant production process whereas other subjects become zombies. Multiple Tyrant types have been produced, most notably the T-103 mass-production model which has remained in circulation on the black market since Umbrella's collapse.
 * Nemesis T-Type - Arguably the most powerful and effective bio-weapon Umbrella has ever created. This Tyrant model was created by introducing an NE-α parasite to a T-103, granting it increased intelligence, allowing it to wield different weapons and tools and carry out more advanced commands.
 * T-A.L.O.S. - A Tyrant model augmented with advanced cybernetics and coordinated by the U.M.F. -013 supercomputer. This was the last of Umbrella's major bio-weapons before the company was dissolved.
 * Chimera - An experimental B.O.W. created by combining human and fly DNA using the T-virus as a catalyst. The creatures proved to be uncontrollable and deemed as failures.
 * Yawn - A giant snake B.O.W. encountered by the S.T.A.R.S. during the Mansion Incident.
 * FI-3 Neptune - An experimental shark-based B.O.W. produced at the Arklay facility. The great white shark only underwent minimal mutation after the administration of the T-virus and was deemed a failure.
 * Evolved Licker - An advanced version of the Licker produced at the Umbrella labs beneath Raccoon City.
 * Plant 43 - Also known as "Ivy". Plant-based B.O.W.s that proved to be too costly to produce and could only move at a very slow pace. Enhanced versions of these creatures would be created later on by introducing human DNA.
 * Plague Crawler - Early insectoid bio-weapons created by James Marcus.
 * Eliminator - Early mammalian B.O.W.s produced by injecting monkeys with the T-virus.
 * Lurker - An early attempt at producing an amphibious B.O.W. by injecting the T-virus into a frog. The specimens grew to a great size, capable of swallowing humans whole, but could not be controlled.
 * Stinger - A scorpion injected with the T-virus. Though it increased in size and ferocity, the scorpion developed a brittle carapace around its head and lost the ability to produce venom.
 * Albinoid - A salamander-based B.O.W. capable of generating high-voltage electricity. It was deemed a failure as it could not be coerced to leave its aquatic environment.
 * Bandersnatch - A humanoid B.O.W. that sported a highly elastic arm, granting it increased attack range.
 * Gulp Worm - An earthworm that grew to enormous size after being injected with the T-virus.
 * Black Widow - An arachnid bio-weapon produced on Rockfort Island.
 * Nautilus - A failed B.OW. that essentially served as an organic turret, producing living projectiles called Torpedo Kids.
 * Nyx - An amorphous creature that relentlessly consumed and assimilated organic matter, increasing its own size and power.
 * Pluto - A bloated humanoid creature with heightened agility despite its obesity.
 * Anubis - A creature created by splicing the DNA of a bat with insect genes. Extensive surgeries carried out on the specimens have left them with absolutely no body fat or muscle tissue, yet this skeletal creature is both extremely agile and durable.