THIS DISEASE TURNS THE LIVING TO ZOMBIES!

Imagine waking up, one cloudy morning and you realise that you're dead, the world around you is faded and you have a fake dead life imposed on you, forcing you to live fake reality... like  a hell where you see everything you love but deep down you know they're not real...

Well folks, that's basically a light description of how suffering from Cotard syndrome feels like.
It's a rare  delusional psychosis which makes individuals suffering from it believe that they are dead, not existing, has lost his/her organs and cannot be convinced otherwise, no matter how much evidence presented to them as proof that they are not living in an apocalyptic hell. It's also known as Cotard delusion and walking dead syndrome.
It's not a new disease, it dates back to the 1700, and was first described by Dr Jules Cotard in 1882. Since the patients already believe they are dead, they don't care about their physical health, they don't care about hygiene, and in some cases, they don't eat and so eventually starve to death. I mean if you're dead what's the point of food right? (But wait... don't zombies eat brains?)
According to research, it's due to the malformation of the  fusiform gyrus and amygdala in the brain responsible for recognition and emotions.
So far there's no real cure for the illness but some treatments such as the use of antidepressants and antipsychotic drugs; electroconvulsive therapy has proven to be more effective than the pharmacotherapy.

Side Note thought; What if the walking dead series living characters all suffered from cotard syndrome gone rogue and they were just a bunch of serial killers who don't know what's real, killing the zombies who they think are dead but are actually the normal ones who are trying their best to stop them from destroying the world.