Could zombies exist

Answer 1

It's rather implausible, but it is entirely possible, at least in the "infection" sense, rather than the "dead rising from their graves" sense.

It is not entirely implausible that a bacterium or virus might mutate in such a way that it infects human brains and overrides, cuts off or destroys higher brain functions, leaving some of the more primitive systems intact, like recognition patterns, the drive to eat, motor control, etc.

Some of the chemical processes associated with death may result from the disruption of normal metabolism or brain function, such as the onset of rigor mortis and decay. Such would result in the lurching, stiff-legged walk often associated with zombies in fiction (e.g. Shaun of the Dead, as opposed to the fast, vicious zombies of 28 Days Later).

This could be a complex, unlikely mutation, or it might be rather simple, there isn't really any way to say. The benefit of being able to spread quickly to new hosts (although the overall infection might be rather short-lived, as zombie populations die of starvation) would be very advantageous, and of course, unwitting, like when cold viruses cause sneezing, which helps spread them to new hosts.

Something like a zombie infection is already taking place in some ants, in the form of the Lancet Fluke. This parasite actually infects the nervous system of the host ant, causing behavior suicidal to the ant but necessary for the reproduction of the fluke. In principle, then, zombie ants already exist.

Answer 2

No, Zombies as traditionally depicted in popular culture - reanimated corpses with a hunger for human flesh - do not exist. The concept of zombies is purely fictional and not supported by scientific evidence.