Doctor James Marcus succeeded in creating the T-Virus at the Arklay Labs on December 4, 1978 through synthesis of the Progenitor virus and leech DNA. Through this synthesis, the photosensitivity of the early Progenitor strain was replaced with pyrosensitivity. The research and development of the virus have become a key issue in Resident Evil history. It truly began showing importance upon the death of Marcus, when Drs. Albert Wesker and William Birkin stole the research to initiate development of a virus of mass destruction.
The T-Virus operates similarly to most other viruses in the Resident Evil series, but also has the abilities to infect dead tissue (differing from real-world viruses), and to power corpses back to life by continuous destruction of the neuronal networks and mitochondria in cells, decreasing motor and higher brain functions, leaving only enough intelligence to allow for feeding and propagating the contagion.
The energy created by these acts damages the host body's response capabilities, and often makes it slow and mindless; many times it is also driven into a furious rage. It has the capability to substantially mutate its host as well, and to infect any tissue in virtually any type of host; creatures as salamanders and elephants have been affected the same way as human beings, and even crows and lions are viable carriers.
By the mutagenic process of the virus, most of the body's 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 casually-infected Bio-Organic Weapons. The mutations are produced when the virus incorporates itself into the host's RNA and considerably alters it.
Creatures with genetic structures different than humans generally show less severe mutations, and usually only increase in size and aggressiveness. This is due to a side effect of the virus' cannibalization of its host, specifically its digestion of the host's frontal lobes. The virus also damages the hypothalamus, which results in a flood of neurotransmitters, enzymes, and hormones which induce a psychopathic rage and hunger in the host.
The T-Virus and its various strains have been used to infect countless walks of life in the past. As it often the most transmissible of the Umbrella Corporation's biological weapons, many lifeforms are accidentally infected upon each new outbreak; even plants have been known to evolve horrifically upon exposure to it. Insects often increase to many times their original sizes, while virtually all infectees develop a taste for a carnivorous diet or at least a blood-based one. Other random, bizarre mutations are known to occur, but most often are one of a kind, and thus are unlikely to be ever repeated.