Okay, so I don't know that this hasn't been developed elsewhere. But for the moment let's imagine that if Hollywood wants to update the vampire and zombie mythologies for our age they gotta come to me! Here's the elevator pitch.
Nano technology is injected into the blood streams of those who can afford it. The self-reproducing nanites fight illnesses of all kinds by low-level sensing of their environment - shared with external computers - and programmed attacks that are controlled from those external computers. This works well for years, until all who can afford it buy into the technology.
Slowly the nanites evolve away from what was injected into the blood stream. At some seemingly sudden point in their development, the built-in self-destruction mechanism is deactivated and the connection to external computers is cut. The nanites develop a new imperative: protect the hive. All those years they survived in the blood stream through the consumption of the energy brought by blood to the cells. They now develop a taste for red blood cells themselves, because of their richness. The nanites also learn to communicate their need simply: you will suffer if you do not replenish the blood cells. The first human hosts develop a network in which they communicate about these changes and learn to consume blood. The blood cannot go into the stomach, however; it must come directly into the blood stream.
The first vampires arise from a group that has their mouths surgically altered to allow the sucking of blood directly. Their vampirism is not lost on them, but their brains have also been affected by nanites, and they have lost any compassion for unaltered humans. As they feed the nanites and those artificial creatures become more dominant in their bodies, they change in other ways as well. They become stronger and quicker; they feel little or no pain. They also gain a sensitivity to light because of an as-yet still functional limitation to the nanites: they cease to function in UV light. This was programmed into them to prevent their casual spread (and protect the corporate profits of the companies selling the nanite treatments).
Zombies, you ask, what do vampires have to do with zombies? If the altered human does not get enough red blood injected into his or her blood stream, the nanites change into a more aggressive and desperate mode. They invade the brain and seek to control it. They don't have the intelligence to control the human mind, but they are able to stimulate the baser parts of the brain. Violence becomes the norm, and if the body is not already half dead from their predation it soon becomes torn-up, infected and wasted. Fine motor control is lost, as is speech. Eventually only the nanites keep the body moving, as they consume it from within.
The nanites in the brain stimulate the desire for - brain. The brain cavity gets direct communication with the mouth so that the nanites can bring any brains or other protein consumed right back up to the skull.
At the same time, whenever the zombie's mouth chomps into something, it leaves behind nanites. The deposited nanites are on high alert and consume and reproduce as fast as they can to start another hive.
Can these vampires and zombies be killed? A stake to the heart or a bullet to the brains? The nanites are not a substitute for organic life; they are parasitic. When the body finally gives out completely, they may seek to abandon it, but even so they are doomed to die for lack of a suitable energy source. A stake to the heart may, in fact, be too traumatic for the nanites to overcome. A bullet to the brain may prevent the nanite stimulation of instinct.
These vampires and zombies do not rise from crypts or graves. They can regenerate a human who is clinically dead, but they cannot work with a body whose cells are decayed.
Are they sufficient to overrun society? The problem is that the disease is already widespread before it manifests itself, and is widely distributed in the most important quarters of society. We don't realize how deep in the shit we are until we have met the enemy - and we realize they are us.
