Gray Death

The Gray Death is a nanovirus used by Majestic-12 in the game Deus Ex as part of their plan to take over the world

Design
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 thus the cells they have integrated with.

The nanites were originally created by Morgan Everett but the idea to use them as a weapon was first put forth by Bob Page.

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.

Use
MJ12 used the Gray Death as a way to blackmail governments as Versa Life, a company run by them, was the only source of the Ambrosia vaccine. One example of this is when putting a Senator on the priority list for Ambrosia is used by Bob Page to get Walton Simons appointed as director of FEMA. MJ12 also used the virus to create a states of emergency and generally destabilize the world.

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 nanoaugment anyone in the process.

Bob Page hints towards the end of the game that any cure will be ineffective as he can simply change the Gray Death architecture using a high prime number to "encrypt" the virus. This could be interpreted as Page becoming desperate knowing that he is vulnerable.