The first and foremost thing I will discuss is why this is not only possible, but entirely plausible. First off, there are a couple of words you should know the exact meaning of, such as mutation and evolution. In the case of mutation, it's a change in an organism's genetic structure due to outside influences such as environmental necessity or radiation. Where it is unlikely that radiation would cause a significant enough change in a virus to entirely change its host, environmental change isn't at all far fetched.
This, of course, leads us into evolution. Evolution, like part of mutation, is attributed to necessity. Usually occurring at exponentially slow rates, this is not the case for viruses and bacteria. We are focusing on the virus, because of it's already zombie like qualities of spreading and, in a few, inducing behavior. As I said in my last post, the most likely candidate is a rabies like virus, for it already instills within it's host zombie like qualities. Aggressiveness, loss of thought process and the total takeover of primal instincts; not to mention the way it spreads. Through transfer of fluids. Sound familiar?
Back on our initial topic, the evolution of viruses. Viruses evolve at an alarming rate out of necessity, because they must evolve faster than their host cells or otherwise risk extermination by the host's immune system. A virus must keep a steady host to continue reproduction for obvious reasons. This means that if a host is limited or extinct, the virus must adapt to a new host.
Viral evolution has already been proven, most predominantly in the influenza strands. Some notable flu strands are the H1N1, which caused the Spanish flu back in the 1800s (now commonly referred to as "swine flu"), H5N1, referred to as the "Bird flu" (which, notably,is a current pandemic threat there is a fear of adapting to humans) and H7N3, which is also referred to as bird flu (though not as severe as H5N1). These strands of the influenza virus had adapted to their current ecosystem, as they were incapable of moving to a new one.
Now let me ask you this. Cities are growing at an alarming rate, destroying many forests, and heavily wooded areas. Common carriers of rabies are foxes, bats, rats and dogs(wild and domestic), however most of the animals that carry the virus live in these areas. Now, it must be noted that it does not take a significant environmental shift to induce adaptation. I"m not saying that all the wooded areas in a certain location must be destroyed for adaptation to occur. Viruses are much to cautious for that, instinctively so to speak. If a significant enough shift takes place, the virus will adapt accordingly. In the case of rabies, adapt to the most abundant mammal in the area, which would undoubtedly be human beings.
We have a potential epidemic on our hands and most people refuse to see it, be it out of fear or because the cinema makes it look like something that can't happen.
If there are any questions, feel free to ask them in the comments, but for now I'll have to leave. I'll answer all questions I can in a separate post and will probably keep a unique post continually edited for questions. My next article will be for survival, so don't forget to check it out and comment. I'll try to help everybody's odds of survival, so stay tuned.