This is how it will go down.
In roughly 25 years, scientists will perfect nanomachines, and human/robot integration. We will be able to successfully fuse robotic components into humans and nanomachines will help us control these components. From here we have a company who decides to sell communication devices which will be implanted into us. For example, we may have cellular phones installed into us. This will create a computer network between the physical bodies of humans.
Now mind you that we also will have the ability to install limbs and organs into our body, and possibly upgrade them. If I wanted to have robotic arms because it would make my construction job easier than I may be able to get those installed. Also our minds will have the same functions they do now, along with the functions of a PC (or Mac). Once we establish a network, we may be able to run a google search with our minds and access a tremendous amount of knowledge, and store it efficiently within our mind. These possibilities are amazing and have to potential to create a utopian society; however, underneath this I foresee a terrible future.
Fast forward 50 years from now a large portion of people will have this technology built into them, for at that point it will become widely accessible and relatively cheap. At this point, governments may attempt to develop a computer virus for military purposes. This virus can infect the cybernetic cells within the humans that occupy them and possibly gain control, or completely render them useless, much like computer viruses today. The virus wouldn't be airborne or spread via contact, it would spread via address books and internet connections. Making the population of infected grow exponentially. Eventually humans would die, either from natural causes or this virus, however, the infected cybernetic components would still be active, because they are being run from an outside source.
This is where nanotechnology comes into play. Nanomachines, as described earlier in this site, can create matter on a molecular level, by re-arranging atoms. Potentially these machines can recreate a physical body. This physical body can then have all the function of a walking talking human, minus one thing, a free will, or (in spiritual terms) a soul. This creates a zombie.
A zombie by definition is "1 : usually zombi a : the supernatural power that according to voodoo belief may enter into and reanimate a dead body b : a will-less and speechless human in the West Indies capable only of automatic movement who is held to have died and been supernaturally reanimated" (Merriam Webster) However, these zombies will have a different supernatural quality, robotics. Robots, or cyborgs, by definition are super-natural, or not-natural.
These zombies will be able to communicate with each other via the wireless network previously established and have the potential to destroy the human race.
As far fetched as this seems, this is a very dangerous thing to be playing with. "singularity" or "techno integration" should not be toyed with, or if it is, it should be heavily regulated to prevent any thing even close to what was just described.