Cat ladies, cockroach slaves, and zombie ants: What do they have in common? They're all probably the unwitting victims of brain-hacking parasites.

Just about every living thing plays host to parasites, including parasites themselves. Much of the time, the host won't even notice the many little freeloaders that quietly feed and reproduce inside it without causing much of a fuss. But some parasites have more dramatic ways of letting the hosts know about their presence and even who's running the show.

Your Cat Really Could Be Driving You Crazy

Toxoplasma gondii is a single-celled animal with a two-stage life cycle. During its sexual period, T. gondii lives and reproduces inside the digestive tract of a cat. The newborn parasites are expelled in the cat's feces and passes on to another host, usually a rodent. Inside a mouse or a rat, T. gondii sets up shop in the brain and muscle cells and begins to reproduce asexually, forming cysts in the host animal's tissue.

But T. gondii needs to find its way back to a cat so it can reproduce. To do this, it has found a way to exploit its position in the brain to make mice and rats attracted to a cat's scent, instead of fleeing from it. Scientists believe it does this by secreting an enzyme called tyrosine hydroxylase, which catalyzes the formation of dopamine, a neurotransmitter integral to reward and pleasure pathways in the brain.

This parasite doesn't need to learn how to make neurons act as if they are pleasurably anticipatory; it takes over the brain chemistry of it all on its own, Stanford University researcher Robert Sapolsky said in a 2009 interview with Edge magazine.

Rats and mice aren't the only creatures infected by T. gondii: The U.S. Centers for Disease Control and Prevention estimates about 60 million people in the U.S. alone are infected.

And the parasite is thought to influence human behavior as well. T. gondii has surfaced in news headlines at least twice this year -- most recently, on the the heels of a study that found a link between the parasite, which commonly finds its way to humans via cat feces, and a higher risk of suicide in some women.

Other research papers have linked T. gondii to mental disorders such as schizophrenia, as well as higher rates of traffic accidents in infected men.

It's not crazy to think that the same effect T. gondii has on rodents could be linked to the behavior of cat ladies -- women who hoard way too many cats and seem unfazed by the scent of cat urine.

There isn't any hard evidence proving this, though: You'd have to catch someone prior to T. gondii infection and prior to hoarding cats, then see how they behave after they are invaded by the parasite.

The Suicidal Fish

The strategy used by T. gondii -- manipulate one host in the hopes that it will get eaten by the next -- is surprisingly common in the parasite world.

Euhaplorchis californiensis, a parasitic trematode flatworm, actually cycles through three hosts: birds, snails, and fish. E. californiensis displays its mental manipulation skills when it invades the California killifish. The flatworms first wriggle in through the fish's gills, then follow nerves back to the fish's brain.

Infected killifish are four times more likely than uninfected fish to make conspicuous displays -- shimmying, jerking, flashing their sides, and surfacing -- according to researchers. This makes them 30 times more likely to be eaten by a bird, thus passing the flatworm onto the next host.

Jenny Shaw, who studies the killifish-trematode relationship at the University of California at Santa Barbara, said it's a bit difficult to study the effects of infection on killifish out in the wild because the parasite is so successful: Infection rates are about 100 percent in most killifish locations throughout Southern California.

What we do know is that, like T. gondii, E. californiensis seems to affect the reward pathway in the host brain, possibly by manipulating levels of dopamine and another neurotransmitter, serotonin.

But the explanation for an infected killifish's seemingly suicidal behavior might not be as simple as the racheting up of a brain chemical or two.

It's difficult to pinpoint the exact mechanism, because there are so many different physiological systems that help determine behavior, Shaw said in a telephone interview. It's not just dopamine that's involved, but also the immune system and the entire neuroendocrine system.

Death March Of The Zombie Ants

You might not think that a fungus is all that threatening. But Ophiocordyceps unilateralis is a stone-cold killer of carpenter ants, and it manipulates its hapless victims in an especially grisly way.

Once the fungus infects the carpenter ant, it allows its host to remain alive for a time, but only to take one last walk. The fungus pilots the ant down from its nest near the treetops into the understory of the rain forest and the underside of a leaf. The ant, still controlled by the fungus, bites down on the leaf and dies.

Meanwhile, the fungus reproduces inside the ant's corpse. A few days after the ant dies, the fungus' reproductive organ, called a stroma, erupts from the ant's head and sheds spores toward the forest floor, each in search of another unlucky ant.

In 2009, Harvard University researcher David P. Hughes and his colleagues found that the infected carpenter ants were always latched onto the undersides of leaves 10 inches above the ground and almost always on leaves growing from the northwest side of the plant.

In a paper in the American Naturalist, Hughes and his team described how the parasitic fungus failed to flourish when they placed infected ants in different locations. O. unilateralis apparently has specific requirements for humidity, temperature, and sunlight.

The fungus accurately manipulates the infected ants into dying where the parasite prefers to be, by making the ants travel a long way during the last hours of their lives, Hughes said in 2009.

Come, Cockroach Slave!

Ants aren't the only insects that can get zombified by parasites. Cockroaches in tropical regions have much to fear from the emerald cockroach wasp, which, like many of its relatives, stings its victims. But this wasp has a dastardly secret: It aims for the brain. Plus, the venom of the emerald cockroach wasp doesn't fully paralyze the cockroach, but stupefies it. The victim can still walk, but it can't move under its own power.

After stinging its victim, an emerald cockroach wasp will take hold of one of the cockroach's antennae and take it for a walk like a dog on a leash, Ben-Gurion University researcher Frederic Libersat told the journal Nature in 2007.

The wasp leads the doomed cockroach back to its nest, where the master lays an egg on the helpless slave. Once the egg hatches, the larva eats the still-stupefied cockroach.

Lest you be worried about some sort of mutant emerald cockroach wasp swarm stinging you and turning you into a docile baby wasp buffet, Libersat and his colleagues have found an antidote to the wasp's zombie-making venom.

In a paper in the Journal of Experimental Biology, they described how they found that the venom works by blocking the neurotransmitter octopamine. The researchers were able to help cockroaches shake off the venom's effects by giving them a compound to switch their octopamine receptors back on.

These parasites are just a sampling of the hundreds of protozoa, insects, worms, and other creatures that have found mind control to be the best survival strategy.

Comparatively, humans have gotten off pretty easy in the parasite department -- we don't fall victim to barnacles that force us to undergo sex changes, like some crabs do. And thanks to our position at the top of the food chain, we're a dead-end host for those parasites that rely on inducing suicidal predator-seeking behavior.

So if you end up with an advanced case of toxoplasmosis, schizophrenic, and surrounded by hundreds of stinky cats, just remember, things could be worse!