Invariably, zombie fiction is dominated by settings wherein society has collapsed and The End Of The World As We Know It confronts the characters we're supposed to identify with. It is, therefore, simply a matter of substituting a real-life threat (such as criminal gangs) in place of the shambling Undead in our minds, in order to lend some credence to what is unfolding on the screen or page in front of us. This is why Survivalist literature of the past often referred to generic threats, called simply MZBs, which stood for Mutant Zombie Bikers and represented the roving threat that they expected would come to terrorize the countrysides of America in a societal collapse scenario, once law and order ceased to be a deterrent.
But here is an interesting question: Is a real-life Zombie Apocalypse possible? The answer may surprise you, because it's more complicated than you might think.
Strictly speaking, the classical idea of the zombie as a so-called Undead creature that reanimates the human body after a person has passed on, is purely -- as I'm sure you expected -- an element of fiction. However, there are circumstances wherein, if certain diseases made a small genetic mutation (as they are often wont to do), we might suddenly find ourselves facing a situation not terribly unlike the one depicted in the film "28 Days Later" [Blu-ray] (DVD version) and it's sequel "28 Weeks Later" [Blu-ray] (DVD version).
All tangent effects remain the same, however: society collapses in areas where the virus takes root (England, in the case of the films) as ravenous bands of violent monsters that used to be people roam the countryside. Obviously, that makes for a much more realistic scenario; and, worryingly, it isn't entirely out of the question as something that could really occur.
Take, for instance, Creutzfeldt–Jakob disease (or CJD). This is a degenerative brain disease in which the subject develops holes in their brain tissue, leaving it looking like a sponge. It is often erroneously referred to as a human form of Bovine Spongiform Encephalopathy (or BSE), commonly referred to as Mad Cow Disease, though this is incorrect. The two are not related. Where things become more muddled, however, is that BSE is thought to be the source of a variant of Creutzfeldt–Jakob (Brown, 2001) that is designated vCJD.
The scary thing is that, once infected, death is certain. There is no cure. It is only a matter of time, during which the patient's brain cells die, causing them to suffer from a dementia that progresses rapidly, causing memory loss, hallucinations (which are sometimes violent), and changes in personality. Problems speaking follow, along with -- eerily -- other physical problems that make the zombie comparison even more pronounced: their movements become jerky; and, they become uncoordinated, shambling and stumbling as much as walking.
As stated previously, there is growing scientific evidence (Collinge et al., 1996; Bruce et al., 1997) that human beings can become infected through ingesting animal products that are tainted by Mad Cow Disease. In fact, some folks in Kentucky became infected in 1997, and the thought is that they may have contracted the disease from consuming the brains of squirrels (Berger et al., 1997).
Also, and perhaps even more worryingly, Neuropathologist Laura Manuelidis of Yale University and several colleagues published findings in 2007 that reported having identified virus-like characteristics in a small number of test subjects (Manuelidis et al., 2007). As it stands, Creutzfeldt–Jakob disease is fairly rare, but nothing with regard to viruses can ever be counted upon to remain static forever.
In the end, all that is truly needed is for the disease to mutate into a form that is more virulent and, therefore easier to pass from human to human, and we could truly find ourselves in the midst of a real-life Zombie(esque) Apocalypse.
Berger, J.R., Weisman, E., & Weisman, B. (1997). Creutzfeldt-Jakob disease
and eating squirrel brains. The Lancet, 350(9078), 642.
Brown, P. (2001). Bovine spongiform encephalopathy and variant
Creutzfeldt-Jakob disease. Bmj, 322(7290), 841-844.
Bruce, M.E., Will, R.G., Ironside, J.W., McConnell, I., Drummond, D., Suttie, A.,
... & Bostock, C.J. (1997). Transmissions to mice indicate that ‘new variant’ CJD is caused by the BSE agent. Nature, 389(6650), 498-501.
Collinge, J., Sidle, K.C., Meads, J., Ironside, J., & Hill, A.F. (1996). Molecular
analysis of prion strain variation and the aetiology of 'new variant' CJD.
Manuelidis, L., Yu, Z. X., Barquero, N., & Mullins, B. (2007). Cells infected with
scrapie and Creutzfeldt–Jakob disease agents produce intracellular 25-nm virus-like particles. Proceedings of the National Academy of Sciences, 104(6), 1965-1970.