The next truly revolutionary epoch of human history will be the building of successful nanotechnology. It will redefine medicine and surgery to levels that will be unimaginable. A nanodrone guided by an external operator will be able to kill cancerous tumors, remove potentially fatal blood clots and fatty deposits and repair damaged tissue. More specialized nanobots programmed to home in on certain protein sequences can identify and destroy invading pathogens, or engage in even more advanced tasks such as rebuilding neural pathways damaged by illnesses like Alzheimer's.
Such an innovation will be a red letter day for human civilization. However, it will also mark the day when a new doomsday device has been created that can annihilate everything that we have. A rogue nano agent, even if it cannot self-replicate, will be far worse than the most virulent cancers and hemorrhagic fevers that exist today. And the ability and ease for rogue governments and terrorist organizations to get their hands on a stock of nano agents and program it to do their willing is what makes it even more alarming. A nanoagent caused plague, often referred to as a "grey out", may have first been inadvertently described by HP Lovecraft in his early 1900's extraterrestrial horror story The Colour Out Of Space.