Plaga (Resident Evil: Undead)

Viruses are microscopic particles that can infect host cells in ''Resident Evil: Undead. ''Viruses are incapable of replicating without infecting a cell and at their most basic, they are genetic material within a protective protein.

T-Abyss Virus
The T-Abyss Virus was developed by the II Veltro terrorist group when conducting deep-sea investigation at the Kermadec Trench with an unnamed exploration vessel. On their fourth expedition, reaching the Hadopelagic Zone, the group discovered a new Virus in the body of a deap-sea fish, a Virus that allowed the fish to survive in such great depth and they dubbed it the T-Abbyss Virus, after the fear of unfathomed depth.

The creation of the T-Abyss Virus demonstrated the ease of developing Viruses that can mutate across species and mammals were just as susceptible to it's effects. Although there were only a few incidents the group used the Virus in attacks, new types of B.O.W.'s were quickly developed with every use of the new pathogen.

Progenitor Virus
Progenitor Virus was an ancient West African Virus which was the basis of the bioterrorist agent known as the T-Virus. As the Progenitor Virus lays at the heart of international bio-weapons development, it's original genetic material has been artificially-altered in many ways to suit the researchers' needs. In 1967, the Virus has been known to exist in distinct strains: Type A and Type B.

The Type A was tested on Jessica Trevor on November 10 1967. The Virus failed to fuse with her DNA and she was later killed. It was noted that she did not simply die from Progenitor, which were the fates of Marcus' Human test subjects. Furthermore, she managed to live for three days, and with a reasonably good-condition brain. Her mind was not fully intact, however, describing drifting into unconsciousness and memory loss in a letter.

The Type B was tested on Lisa Trevor the same days as Jessica's testings. This strain of the Virus succeeded in fusing with the DNA, albeit with a displayed reaction. Her brain either due to the Virus strain or her young age was significantly affected by the Virus, killing a victim and losing the ability to write by November 17.

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