They are everywhere, and of course, nowhere.
Zombies are hot commodities on television, in video games and on movie screens. Yet they are merely dark fantasy fit for horror stories and Hollywood thrillers.
But in the spirit of Halloween, we ponder what if and ask whether the world could face shambling armies of the undead.
With tongues planted firmly in cheeks, we asked scientists with a fascination for the macabre to explain how human beings could change from hard-working members of society to mindless cannibals and trigger a real-world zombie apocalypse.
"There is some precedent for it in nature," said Tara Smith, a University of Iowa epidemiologist who noted that there are parasites and fungi that turn host insects into zombielike slaves.
In the real world, Smith studies how viruses can mutate and jump from one animal species to another.
She is among a group of medical experts and researchers who serve as advisers for the Zombie Research Society, an organization "dedicated to the serious study of zombie science." Its motto: "What you don't know can eat you."
Zombies have never been more popular. AMC's The Walking Dead draws millions of viewers every week.
There are comic books, video games and a growing list of popular movies that include all types of zombies, from slow shufflers to fast-moving brain-eaters.
But how did they get in that state? Most fictional accounts lack specific explanations. And most are unclear why these zombies are so hungry.
In the 2009 movie Zombie-land, for example, the narrator mentions an infection that sprang from "bad hamburgers."
Dr. Steven Schlozman, a Harvard Medical School psychiatrist and zombie research adviser, offers what he considers a more-plausible agent in The Zombie Autopsies, a work of fiction he wrote.
Schlozman said splicing prions to the tail of an influenza virus could do the trick.
Once a person breathes in the virus, the prions, similar to those found in mad cow disease, could eat holes in the frontal lobe of the brain, causing problems with decision making and motor control. A third viral agent, Schlozman said, would then attack the hypothalamus, making the "zombie" feel hungry all the time. Schlozman calls his disease ataxic neuro-degenerative satiety deficiency syndrome. Such a virus, he said, would have to be engineered.
"We couldn't think of a way that this on its own would evolve to get everything together -- that stumbling walk, the poor cognitive ability and the constant hunger," Schlozman said.
Bradley Voytek, a University of California/San Francisco neuroscientist who studies internal brain communication, said the damage to the brain would need to be more severe.
He said the cerebellum, which controls motion and coordination, and both hippocampuses, which regulate memory, would have to be damaged.
"(Zombies) clearly have memory problems," Voytek said. "They don't seem to remember family members and things like that."
Voytek calls his zombie-making disease consciousness deficit hypoactivity disorder, but he won't hypothesize how folks would contract this malady.
Smith said another potential zombie agent is rabies.
"That's probably the best biological model," she said. "It does cause aggression. It does cause animals to bite each other, and it is spread through saliva."
Rabies, however, is a slow disease.
"If you get bit somewhere close to the head, you would develop symptoms in a couple weeks," Smith said. "In the leg, it's much longer."
That leaves plenty of time for government officials to corral the infected and stave off anarchy.
The chances of a fast-moving, mutating virus are slim to none, said Richard Slemons a professor of veterinary medicine at Ohio State University.
Slemons studies how strains of human and animal influenza, which does spread rapidly, can trade parts to form hybrid viruses. He said it's nearly impossible for rabies and influenza to join forces.
That's because they've evolved to attack different types of host cells. Influenza sets up shop in the lungs, and rabies prefers the spinal cord and brain.
There is one thing the zombie researchers agree on -- corpses can't turn into zombies.
"They can't rise from the dead," Schlozman said. "Nothing reanimates."
But let's just say, for Hallo-ween's sake, that something remarkable like that could happen, that there is a virus that attacks humans, acts quickly and turns us into zombies.
If that happens, Philip Munz, a University of Western Ontario graduate student says, humanity won't have much time. The mathematics student wrote a paper in 2009 that offered a model for the spread of a zombie epidemic.
"It would be about eight days for a city the size of Ottawa, which has about 1 million people, to be overwhelmed by the shambling hordes," Munz said.
That's about the same population in Franklin County. Sweet dreams.