Honestly, you take me entirely too seriously.
Parasites are just as likely as any other method of zombie infection. In essence, zilch.
However, blending realism and fantasy together allows for fun and creative ideas, as believability lends itself quite nicely to immersion. You don't actually have to believe that any of it is true, your humanly irrational side will take care of that for you.
This is why we can watch a scary movie, even an absurd or fantastical one, and get the heebie-jeebies. Even if the killer/monster dies (without any room for sequels), we're still put on edge. This is why most people watch scary movies in the first place, to get a nice spooking.
For instance, parasites. Certain parasites, that have been engineered by evolution to be able to do it, are capable of taking over a slug's brain and causing it to lay about on a leaf with its parasite-swollen eyestalks waving around like beacons. A bird comes by, eats the eyestalks, and gets the parasites into its system.
The bird takes a parasite-filled dump, a slug crawls over the splatter, and the cycle continues.
Would a parasite be able to take over the absurdly complex human mind? I seriously doubt it. It would have to be some genetically engineered super worm that shoots flames from its eyes. but you don't have to know that, all you have to know is that there are naturally occurring parasites in the wild that are capable of taking over the nervous systems of minor animals.
Point is, zombies ain't happening. That doesn't mean we can't have fun speculating about interesting ways of creating background for a theoretical zombie-infested world where we can fulfill our lifelong dreams of smacking someone with a frying pan without it being illegal.