The Gray Death occurs when an individual without a predisposition for nanoaugmentation is infected with nanites. The nanites integrate themselves into the individual's cells and the individual's body rejects the nanites, and by extension the cells that the nanites have integrated with. The virus can be halted by the use of Ambrosia, a vaccine that consists of ROM modules which attach to the nanites. The virus and vaccine both must be manufactured in a Universal Constructor.
The nanites were originally created by Morgan Everett. The idea to use them as a weapon was first put forth by Bob Page; when the proposal was frowned upon by the other Illuminati, he organized a successful coup d'etat against the Illuminati using its splinter faction, Majestic 12.
|"NEW YORK -- No student of recent history can fail to miss the irony that the greatest enemy mankind has faced in the 21st century was not any despot or dictator, but the lowly microbe. Millions upon millions have been killed by the pandemics of AIDs, resistant tuberculosis, and the Century Flu, and now we face our greatest threat yet: the "Gray Death," a plague of devastating efficiency that has surfaced in the last year.
But was this plague an accident of nature, or a design of science? Dr. Kenji Ishii of the New World Biomedical Health Center in New York thinks not. "My analysis of this plague indicates that it is decidedly unnatural in origin, and in point of fact, may be extraterrestrial." He went on to speculate about those afflicted. "Obviously, a disproportionate number of the indigent seem to be infected, a situation that makes perfect sense if you consider it prelude to an invasion. No more effective way to disrupt the social, political, and military fabric of Earth has yet been devised."
If this is so, then what are our leaders not telling us? And how can we be prepared? Only they know they answers."
|— Gray Death: Look to the Sky, Joe Greene (Midnight Sun)|
Once exposed to the virus, it is demonstrated that beginning signs of infection result in flu like symptoms, usually coughing. As the infection progresses to later stages, the individual turns pale white/gray, hence "Gray Death", and becomes increasingly frail, where speech can also be affected. Eventually, the individual would also experience severe pain throughout the body, due to the virus eating away cells, which would inevitably cause the victim to succumb to death.
|"The Gray Death epidemic is spreading rapidly into rural communities, according to a bulletin from the Center for Disease Control in Atlanta. Until now, the plague has been localized in major urban areas, and there was some hope that a large population concentration was required in order for the disease to spread.
The report states that rural infections now stand at 8.7 percent of area population, up from 2.4 percent six months ago. In urban centers, the infection rate has risen from 22.4% to 28.6% in the last six months. In all regions, the plague carries a fatality rate of 93% within the first 100 days of infection.
The CDC bulletin stated that no breakthroughs have occurred in the search for the plague’s origin, vectors or cure, but that government research programs continue."
|— Gray Death Spreads to Rural Communities, Midnight Sun|
There is also indication that the release of the virus was intended to speed up the nanoaugmentation project. MJ12 had Ford Schick, a scientist they captured in New York, working to integrate the Gray Death into test subjects. If Schick, or any other scientist, were to find a way to eliminate the negative health effects of the Gray Death they would discover how to successfully nano-augment anyone in the process.
Bob Page states that it would be extremely easy for him to create a new virus if a cure to the Gray Death was to be found. Apparently all he would need to do is "find a very large prime number and multiply", suggesting the nanite somehow uses random number generation or encryption.
- In Deus Ex: Human Revolution, while in Detroit at the time of the riots, a homeless man can be heard zealously preaching to others. He predicts that there is "a sickness coming my children, gray and deadly" that "shall come on the backs of 12 kings!" This is a clear reference to the Gray Death and its creators, Majestic 12.