The biological troublemakers.
Here's a list of all the nasty infections the series has spread across humanity like so much rancid Vegemite of death. We'll go into the kind of abominations each one is capable of producing as well.
This was the virus that started it all. Borne from a rare species of flower found only in Africa, this was discovered by the founding fathers of the Umbrella Corporation, who used it as a base for all of their evil genetic research. Its most successful animal implementation was the humble leech which--under the influence of the Progenitor Virus--banded together to form large, horrifying creatures.
Successive outbreaks of the Tyrant, or T-Virus, caused the complete eradication of Raccoon City and the surrounding Umbrella facilities around the Arklay Mountains in the late 90's. Created by James Marcus during his bizarre experiments using Progenitor and leech DNA. This virus can be transmitted by many means and kills its host almost immediately. But then, as internal mutations take place, the corpse reanimates into a braindead shell of its former self, with the sole instinct to feed on any living flesh in the immediate area. This has been known to create zombie versions of humans, animals, fish, insects and even plants. For some reason, T-infected insects grow to immense sizes, large enough to easily capture and consume humans.
Some zombies can undergo advanced evolutions (or devolutions, however you look at it) after prolonged infections or under certain controlled scenarios. One common type of which is the Crimson Head, a re-reanimated zombie after it is initially defeated. It will have enhanced stamina and bloodlust, and will slash at its prey with elongated, razor sharp claws. Hunters and Lickers are advanced T-Virus mutations, the result of splicing various animal DNA with human hosts.
Umbrella's most powerful B.O.W, the Tyrant, is considered by them to be a perfect and completed T-Virus creation, a near-unstoppable creature of brute force which can be programmed to follow instructions. Only one in thousands of human hosts are able to accept the T-Virus and transform into a Tyrant. The odds of finding compatible hosts was so bad that Umbrella eventually began cloning its Russian branch's head developer, who was compatible with the T-Virus, for mass Tyrant production. The Nemesis was a modified Tyrant that was implanted with a mind-controlling parasite for greater control.
Created and self-inflicted by William Birkin. The G-Virus is a heavily mutated strain of the T-Virus which originally arose in the ever-mutating body of Umbrella test subject Lisa Trevor. It accelerates the growth of body tissue, turning its victims into grotesque, cancerous blobs barely resembling the original host. G monsters are known to regenerate lost or damaged tissue, are extremely powerful and dangerous and are often identified by a large superfluous eyeball protruding from some part of their body such as the shoulder or back. William Birkin himself became the most powerful and dangerous G-Virus monster, constantly mutating as regenerative processes covered up old wounds and created new tissues.
Created in the early 80's by Alexia Ashford, the granddaughter of one of Umbrella's founders. Based on the Progenitor and infused with a rare viral strain found in queen ants, Alexia injected it into her father, who quickly became a hulking monstrosity and was locked away. T-Veronica worked too quickly and was too unstable. After working out the kinks in the design, Alexia later injected it into herself before entering a statis chamber for fifteen years. Upon her release, Alexia was super-human, displaying a variety of unique powers including armored skin and blood that burst into flame upon contact with oxygen. She eventually mutated into something of a human/ant hybrid, complete with insect wings and an ovipositor. She was killed by Chris and Claire Redfield.
A type of ancient, spore-based parasite harvested from fossils, this organism was used by a European religious cult known as Los Illuminados. Injected into a host while in a larval egg stage, the Plaga slowly take over the host's nervous system, eventually hindering their sense of reasoning. In return, the infected have an extremely high threshold for pain and will unquestioningly comply with orders given by their master. The Plaga within grows all the while, eventually becoming a self-sufficient Face Hugger-like organism.
Plaga-infected humans have the ability to think and communicate amongst themselves while working towards collective goals. If damaged, the Plaga within rises up and continues to fight. Through his hired gun Ada Wong, Wesker stole a Plaga sample, and through his connections with biological research giant Tricell, engineered them to be much more effective and easily absorbed into new hosts.
The latest virus found in Revelations was developed when a marine university discovered a new species of deep-sea fish. Unfortunately this fish was carrying a virus that was named Abyss, and some nasty fellows from the bioterrorist organization Il Veltro combined it with the T-Virus. T-Abyss victims must constantly absorb vast quantities of fluid, and they'll get it from wherever they can, even the bodily fluids of living humans. They lumber around like jiggly, carnivorous water balloons of death, and upon defeat they will melt away into puddles of gore. The types of new B.O.W creations from this virus strain include the Ooze and the Sea Creeper.