Zombies are everywhere. Parasites mean you could be one too.

In the wild, insects, worms, virus, and fungi turn animals into hyper-specialized zombies. Which ones can take over humans?

Zombies are real, and nature is teeming with them. Fungi take over the brains of ants, wasps paralyze cockroaches—a practice called zombification. “Scientists have found that fungi, bacteria, wasps, and worms do it,” says Matt Simon, author of the new book Plight of the Living Dead. “It’s extremely widespread across the animal kingdom.”

Speaking from his home in San Francisco, Simon explains how a species of parasitic wasp made Darwin doubt the existence of God; why zombies in the movies were originally inspired by the symptoms of rabies; and what parasites get out of their excruciating manipulations.

Nature is full of examples of creatures that take possession of other creatures. Tell us about the emerald or jewel wasp.

A particularly bizarre example is the jewel wasp, a wasp that’s about half the size of its victim, which is a cockroach. The mother wasp grabs hold of a cockroach and drives her stinger in between its front legs. That paralyzes the cockroach so it can’t protect itself from what’s about to happen next. The cockroach can’t flail those legs to stop the wasp from taking her stinger and driving it through the neck and into the brain, where she deposits venom. She has sensors on her stinger that allow her to feel around in the brain for two specific spots that govern locomotion. It’s here where she injects that venom.

When she pulls out her stinger, the cockroach acts as if nothing has happened. It will groom itself obsessively but not move from that spot. This allows the wasp to go off, find a burrow, then come back to the cockroach and bite off its antennae. She then drinks the blood to get back some of the energy she lost from driving the stinger into the brain. She will then latch onto the stubs of those antennae and drag the cockroach into a burrow.

But it’s not so much dragging as it is leading. The cockroach at any time is perfectly capable of flying or scurrying away. Scientists have taken cockroaches after they have been stung in the brain and dropped them in water, which snaps them out of their daze, and they scurry off. The cockroach seems to be willingly following the wasp into the burrow.

After the wasp has dragged the cockroach into the burrow, she lays one single egg in its leg. That egg then hatches into a larva, which begins sucking the blood out of the cockroach’s body. When that has run dry, the larva burrows into the body cavity of the cockroach, and eats the organs that are most vital for the survival of the cockroach, like the central nervous system or the heart. When that is complete the larva spins itself into a cocoon inside the body of the cockroach and emerges as an adult. That is the extremely complicated manipulation of the poor, poor cockroach.

Zombie Parasites

What if, unbeknownst to you, someone—or something—was controlling your behavior for its own nefarious ends? Join photographer Anand Varma as he reveals this nightmare scenario being acted out over and over across the natural world.

Another organism that uses zombification is the Ophio fungus. Take us inside its strange world.

This is a fungus that attacks ants. Not only that, Ophiocordyceps unilateralis sensu lato is made up of several different species. Each of these species only attacks one species of ant. The fungus starts as a spore, falls out of a tree, and lands on a cuticle, or exoskeleton, of the ant. It releases enzymes that dissolve the cuticle and explode into the body of the ant. At this point, the ant is pretty much done for. The fungus then begins replicating, growing throughout the tissues, and ends up making up about half the body weight of the ant, which is incredible when you think about it.

But ants are really good at sniffing out invaders in their territories or colonies. If an ant is acting funny, another ant will grab it and drag it into a graveyard outside of the colony. The fungus, as it’s invading the body, has to somehow escape detection, so it doesn’t make the ant behave all that strangely until it begins directing the ant out of the colony. It doesn’t seem to invade the brain per se. It grows a film around it, probably releasing chemicals of some sort that command the ant to leave the colony.

Even more incredible is that as it’s growing through the tissues, it’s invading the muscles and splitting apart those fibers, probably severing the connections of the neurons. This doesn’t seem to make sense because it has to be able to have the ant mobile. What appears to happen—and this is very early science—is that the fungus is forming its own central nervous system inside the ant. It might be releasing chemicals that mimic the neurotransmitters in the ant’s own body to almost literally pull the strings, like a puppet master, and then guide the ant to an extremely precise position in the forest. This is consistent across infections. The fungus directs the ant out of the colony at about noon and onto a leaf usually about 10 inches off the ground. It orders the ant to bite into a vein on the leaf, which gives it a good perch to then kill the ant and erupt out of the back of the head as a stalk, raining down spores below. And it just happens to position the ant directly above the colony’s trails.

Various species of worm also get inside another creature and turn them into their chauffeurs. Tell us how they achieve this and what dangers they have to overcome.

Worms are a particularly fascinating case because they have complex life cycles. One worm gets into a crustacean called a gammarus, which looks like a shrimp and lives in fresh water. The worm needs to not only get inside that gammarus but also complete its lifecycle in the stomach of either a fish or a bird. This is where the manipulations get super diabolically precise! Depending on the species of worm, whether it prefers the bird or the fish to finish its life cycle, it will command the gammarus in different ways. Worms that get into a gammarus that then get into birds will direct the gammarus to spend more time up at the surface. That gets them closer to the birds but also farther away from fish that are living closer to the bottom, to themselves escape the predatory birds. Other worm species want to get into fish, so they will direct their gammarus to stay closer to the bottom, where the fish are living.

So you can see how these species have evolved with such complex life cycles. But it is, in fact, a very clever way of going about life.

Let’s take a sidebar trip away from worms and wasps into the pop culture representation of zombies. Where did it all start?

What’s so brilliant about all of this is that the zombie that we know and love today is a product of rabies! The rabies virus is itself a manipulative parasite that we don’t necessarily think of as parasitic, which can mind control its host. In fact, it didn’t evolve to target us per se, but other mammals. But because our mammalian brains are relatively similar to things like racoons or possums, it can infect us as well.

It’s a horrible thing to watch. You can find videos on the Internet of people infected with rabies. Once you start showing symptoms—foaming at the mouth is the famous one—you are resigned to death. If you get the vaccine before the symptoms present, you will probably pull through. But almost nobody has survived once the symptoms show.

The zombie of pop culture very much mimics the person infected with a real rabies virus. But it gets more complicated. In raccoons or other mammals that the virus typically infects to complete its life cycle, it not only produces that foaming of the mouth, which is the virus presenting itself in the saliva. It also makes those animals more aggressive, which in and of itself is a behavioral manipulation. It transmits by getting the animal to bite another animal, therefore getting into another body.

But it goes beyond that. It makes the animal not only avoid water but very much fear water, which is probably a manipulation to keep the animal from washing the virus out of its mouth. You can find videos of humans doing the same. They’ll recoil as a nurse presents them water. Somehow the virus is manipulating our behavior to recoil at the sight of water. It’s a terrible sickness but it did give us the zombie that we know today.

So far, we’ve been talking about mostly horrific outcomes. But on Isle Royale, in Lake Superior, a tapeworm known as Echinococcosis helps the local wolves get a good meal. Explain the connection.

This is where behavioral manipulations get fascinating. It’s not necessarily the manipulation of the brain, but of the body. When this worm gets into a moose, it moves into the lungs and forms these large, golf ball-sized cysts. That makes breathing harder for the moose. On this particular island, the moose and wolves are essentially trapped together and have this interesting interplay in the ecosystem. The moose that are infected are less able to defend themselves. The moose slow down, grow weaker and are therefore more vulnerable to predation by wolves. So it’s much more subtle.

“I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae (wasps),” Darwin wrote in 1860. And after reading your book, I have to agree with him. Is there any place for god in this world of zombification? And what does it tell us about ourselves?

[Laughs] I was an atheist before I started writing this book and it only cemented my position. Some of these manipulations are just extremely horrible and excruciating for the host species. That should make us reflect on the way we think about species because we don’t know how widespread these manipulative parasites are. How much are we, as humans, in control of our brains, and how much are we unwillingly ceding to these manipulative parasites?

This interview was edited for length and clarity.

Simon Worrall curates Book Talk. Follow him on Twitter or at simonworrallauthor.com.