The Promethean Nanites, also referred to as the Promethean Plague, the Nanite Plague, the Scourge and numerous similar titles are virus-based artificial swarming biological robots supposedly designed for use as curative systems in the medical field. They should have been capable of healing any ill, repairing any injury. Instead, they turned out to be capable of on-the-spot rewriting of a victim's DNA, doing things from as simple as blending one genetic sequence (say, that of a mouse) with another (human, maybe) and resulting in a viable life-form, to as apparently impossible as creating living beings seemingly made up of otherwise inorganic materials.
Even considering this, they should have been a boon to mankind, the heralding action of a utopian future where pain and injury, and even sexual or species-identity, could be cured instantly. Instead, Disaster was the result.
Nobody knows for certain what the origins of the nanites is. Ostensibly, they were a creation of the (at the time) allied corporations known as RSX and Zephyr. Zephyr was a go-getting civilian-oriented company, while RSX was primarily involved in military contracts and weapons-development. On February 11, 2008, parties unknown released the nanites in mumtiple locations, and explosions rocked the RSX facility in FairField, California where the primary research and production-work was taking place. Within hours, the world was in chaos as people... Changed. Far too many simply succumbed to the imprinted animalistic natures, while others went mad from becoming totally inorganic. Worse, untold numbers of people were simply gone.
It was even worse than that- the nanites were capable of infecting animal and some plantlife as well, leading to disturbing monsters appearing as well as creatures that looked as if they'd once been human, when in truth they'd never been human at all. New york weathered the storm, as did some other cities; the solutions for the surviving areas were as diverse as the locations themselves. New York, for example, is a city factionalized between human and nonhuman residents.
Worst of all was Las Vegas. Something went even wore wrong in the gambling paradise, something that left fewer alive. Known as the 'grey goo' scenerio, The nanites simply broke down all organic material they came into contact into more nanites. The fabled desert oasis is now a dead, disturbing blanket of twitching greyish powder and abandoned buildings.
Of course, multiple investigations were begun, and most are still ongoing. RSX and Zephyr each blame the other for the release, and there are multiple conspiracy theories as to what really happened. One thing is certain- with so many points of release taking place at the same time, the release was planned and deliberate.
It is more than a year later, and while civilization still stands the world is a VERY different place. Many cities are abandoned ruins, and those that remain inhabited are far less populated than they once were. Feral victims of the nanites prowl the streets and woods, and the food supply is... Questionable. Can you be certain where that steak you're eating came from, or worse, from whom? Factions have risen amongst humanity and their new, unhuman kin, and the tensions are strung like high-tension powerlines. Violence is closer to the rule than the exception. However, there are some bright spots.
First, RSX had been carrying out an inoculation program amongst its employees, so there IS a way to prevent the infection. Zephyr, likewise, has created a serum that can aid a victim in maintaining his, her or hir mentality and sanity in the case of nanite mutation, and the nanites themselves aid the healing and repair of their hosts.
The Promethean Nanites are airborne, and are generally present in most, if not all of the life forms in an affected area. For some reason, they tend to form a relatively-globular 'cloud' over the vicinity, which slowly expands as more and more nanites are produced by victims. Mutative infection generally occurs through mucosal contact (blood exposure and sexual contact being the most common) with an individual possessed of an infection. The new victim manifests partial transformations of bodily features as they interact with the previously-infected being. Agents of Zephyr and RSX seem to be protected from passing infections between their members or each other, perhaps due to the treatments each use in their personnel.
The nanites are ever-expanding, producing more of their numbers as they infect more and more victims. They have a tendency to manifest... Disturbing... Sexual modifications frequently, including organs of vastly expanded size, unusual numbers of organs, true hermaphroditism and, oddly enough, occasionally beings that are totally devoid of sexual features.
Theories are that the programming of the nanites is a 'cloud-computer' effect, with the entire swarm communicating constantly with one-another in the globular infection-zone. Thus all of the effects they produce can become standardized over time. However, neither Zephyr nor RSX are terribly forthcoming in information on how the nanites function. Nor have they actually revealed how something so far in advance of the state of the art could be developed.