It seems like almost everyone is fairly big into zombies these days, whether that fandom is limited to catching The Walking Dead every Sunday or actually stockpiling water, ammo, and MREs in preparation for a looming apocalypse. Not all zombies in movies and on TV are created equally, as some are slow and dim-witted like in Night of the Living Dead, while others are intelligent and resourceful, like the ones Will Smith battled in I Am Legend. As pervasive as this theme is in science fiction, what does science have to say about the chances of this actually occuring?

Of course, there are those who take themselves far too seriously to be entertained by the concept of zombies. However, there are some serious and respected scientists analyzing features from pop culture zombies and using them to ask important questions about what is and isn’t possible for the human brain and body. The CDC has even issued a zombie preparedness plan, packed full of ideas to be prepared when the dead start walking. By the strangest darn coincidence, it happens to match the list of what would be useful for other emergencies. However, prepping to blast some zombies is a lot more fun than thinking about hiding in a basement from a tornado, which is why the CDC stuck with it.

Typically, zombies come about after being infected with a virus. How widely a virus spreads is related to how it is transmitted. A virus transmitted through contact with bodily fluids is much easier to contain than one transmitted through the air. This is why the most devastating outbreak of Ebola (transmitted through bodily fluid) has killed 4,877 people during the last 11 months, whereas influenza—which is airborne—kills over 250,000 people worldwide each year. If the zombie virus can only be spread through a bite, it would likely be contained before resulting in an apocalypse.

However, it’s important not to mince words here: the undead zombies seen on the silver screen will never be a real thing. The brain needs to be functional in order to send impulses to the muscles to move, and the heart also needs to be beating in order to circulate oxygen and nutrients. With zombies being dead and all, the heart isn’t pumping, the brain isn’t working, and the body isn’t getting what it needs. Case closed; they’re dead and staying dead.

It might be impossible for zombies to be undead, but what about living beings who possess the same traits? Steven Scholzman, a professor at Harvard Medical School, told LiveScience that the afflictions of zombies are neurological, and not a problem with being undead.

Neuroscientists Timothy Verstynen and Bradley Voytek echoed that sentiment when they teamed up to make a TED-Ed video in which they examine a zombie and use physical clues from the patient to diagnose the cause of the symptoms of the lumbering gait, incoherence, slurred and meaningless speech, and insatiable appetite. Each of these traits is similar to known neurological conditions and can be connected to lesions on a specific area of the brain. Though there aren’t any currently-known syndromes that encompass all zombie symptoms, the duo have coined the term Consciousness Deficit Hypoactivity Disorder to officially describe the condition.

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