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GUY RAZ, HOST:

Tomorrow night, vampires and witches and ghouls will roam America's neighborhoods. And I, for one, am hoping they will behave when they arrive to my house. But anyway, in the midst of all the Halloween revelry, there is actually the possibility that you might run into zombies, real ones.

Sabri Ben-Achour, a reporter of member station WAMU here in Washington, explains.

SABRI BEN-ACHOUR, BYLINE: A few months ago, something terrible happened to millions of flies around Washington, D.C.

MIKE RAUPP: We were getting literally hundreds of reports of these crazy dead flies everywhere - on vegetation, on sign posts.

BEN-ACHOUR: Mike Raupp is an entomologist at the University of Maryland. He says the flies were attacked by a fungus, a mind-controlling fungus. I'm not kidding.

RAUPP: It basically zombie-izes them. It manipulates their behavior. The fly moves to a high point, let's say the tip of a blade of grass, the terminal of a leaf.

BEN-ACHOUR: Then the fly freezes up and the fungus spews more mind-controlling spores into the wind. Now, does any of this sound familiar?

(SOUNDBITE OF MOVIE, "STAR TREK II: THE WRATH OF KHAN")

RICARDO MONTALBAN: (as Khan) You see, their young enter through the ears.

(SOUNDBITE OF TV SHOW, "STARGATE SG-1")

JAY ACOVONE: (as Kawalsky) What are you saying? I got one of those parasites in me?

(SOUNDBITE OF MOVIE, "INVASION OF THE BODY SNATCHERS")

UNIDENTIFIED MAN: "Invasion of the Body Snatchers."

BEN-ACHOUR: But forget Hollywood, OK? Here's a horror film for you. It's actually playing on a laptop in front of Mike Raupp.

RAUPP: Yeah. It looks to me like it's coming out of its butt. That's just wrong. That's nasty.

BEN-ACHOUR: Giant worms are exploding out of a dying cricket that's floating in a swimming pool.

RAUPP: It turns out there are small organisms called hairworms. They begin to reach maturity inside the cricket, and they make this behavior become very erratic.

(SOUNDBITE FROM MOVIE, "POLTERGEIST")

UNIDENTIFIED GROUP: Don't be afraid.

RAUPP: Crickets that would normally kind of move pretty slowly and stay in dimly lit areas actually become attracted to light.

(SOUNDBITE FROM MOVIE, "POLTERGEIST")

UNIDENTIFIED GROUP: Lead us into the light.

RAUPP: What this does apparently is bring them out of their normal habitat and increase the likelihood they're going to bump into a pond. Once they reach the edge of that water...

(SOUNDBITE FROM MOVIE, "POLTERGEIST")

ZELDA RUBENSTEIN: (as Tangina Barrons) Go into the light.

RAUPP: ...they take the suicidal plunge into the water.

(SOUNDBITE FROM MOVIE, "POLTERGEIST")

RUBENSTEIN: (as Tangina Barrons) There is peace and serenity in the light.

RAUPP: This is the point at which the hairworm will emerge from its host, find a mate, reproduce and lay eggs, and complete the life cycle.

(SOUNDBITE FROM MOVIE, "POLTERGEIST")

BEN-ACHOUR: Mind-controlling parasites are all around us. The number of creatures that can be affected by them is...

JANICE MOORE: Oh, it's huge.

BEN-ACHOUR: Janice Moore is a professor at Colorado State, and she wrote the book on parasites and animal behavior. She says some parasites play with neurotransmitters; others with hormones.

MOORE: If you take the world of parasites broadly, we don't know the half of it yet.

BEN-ACHOUR: And on TV, the parasites are pretty blunt, right?

(SOUNDBITE OF TV SHOW, "STARGATE SG1")

UNIDENTIFIED MAN: (as Goa'uld) Release me now or I will destroy you.

BEN-ACHOUR: But in reality, they can be terrifyingly precise. One example that's becoming a little more understood is Toxoplasma gondii.

MOORE: Toxoplasma basically makes rodents somewhat fearless around cats. In fact, it's even more than fearless. There's some evidence that they're actually attracted to the smell of cats and the cat urine.

BEN-ACHOUR: Do you know what happens to rats that like cats? They get eaten by cats. But I mean, this is animals in movies, right? We humans aren't just fleshy spaceships for mind-controlling parasites, right? Well, this is kind of where it does get a little scary.

Bob Yolken is the chair of Pediatric Neurovirology at Johns Hopkins medical school. Remember how some hosts become fearless?

BOB YOLKEN: Studies have looked at accidents - individuals in automobile accidents, both actually drivers and pedestrians - and they have increased rates of Toxoplasma.

BEN-ACHOUR: This evidence is nowhere near conclusive, but Janice Moore says it's enough to make you think.

MOORE: I do think about free will some because I do think about how we're all trapped in our own skins. And to tell you the truth, free will in general, it always amazes me how in the same situation some people will rise to the occasion and be saints and other people will be sinners.

BEN-ACHOUR: A lot of things affect that, Moore says, but we may just have to add parasites to the list.

For NPR News, I'm Sabri Ben-Achour.

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