There are a few ways that zombies COULD eventually be real, although they are very, very unlikely. A fungus, or otherwise infectious organism (but most likely a fungus) could bring the dead back to life as rotting zombies, but it would mean that the fungus would have to have evolved specifically to replace tendons, muscles, and other parts of a dead rotting human body, and to be able to bring a dead person back. However, I have no idea how it would do that... maybe by firing electrical pulses through the brain after fixing parts of it?
Nanobots could be another possible cause for zombies, although they would most likely look like regular humans, and probably not be infectious. Nanobots are a lot less likely than a fungus, however, because we don't even have them yet.
Technically, zombies already do exist, although they aren't flesh-hungry, crazed cadavers. A special flower grows in some parts of the world which, when ingested, will cause the person to obey any and all commands.
