We think of parasites as nature’s villains—thieves and con artists that hack into biology for nefarious purposes. When they find their way into video games, parasites are much the same. Who doesn’t subconsciously want to reach for a crowbar when they see gaming’s most famous parasite, Half-Life’s headcrab?

Villains they may be, but parasites vastly outnumber free-living species. This prevalence apparently extends to the rest of the universe too. In the seminal first-person shooter series Half-Life, the parasites known colloquially as “headcrabs” (or even “headhumpers”) are so effective at taking advantage of other species that they are used as biological weapons against humans. Coupling with a victim’s head, a headcrab first grotesquely morphs the body and then takes over all functions.

How does a headcrab make such a good zombifier, and is there anything like it in nature? To the speculation dome!

VavleTime forum member DarkSide55 has spent an incredible amount of time answering these questions. In a free document entitled “Everything You Wanted to Know About Headcrabs,” he outlines the biology, anatomy, and zombification techniques of all known Half-Life headcrabs. And although many of DarkSide55’s answers are based on fan theories, there is actually quite a bit of good science to them:

Headcrabs control their host through the neurons. The headcrab is both intercepting messages from the human brain, and feeding neurotransmitters of its own into our neurons. The entire reason the headcrab doesn’t require the human brain is because it is acting in stead of our brain: it sends the chemical and/or electrical messages of its own directly into our peripheral nervous system, bypassing the central nervous system; they are using our own nerves against us. It is both amazing and ironic that this creature, this alien lifeform we see as a tiny, shrieking terror that tries to overpower us with brute force leaps is using such a sophisticated method of takeover.

And how a headcrab mutates a human body is no less sophisticated:

The headcrab’s mutagens are a retrovirus. A retrovirus is similar to a normal virus in that it is RNA packaged in a protein envelope, latches onto a cell, and injects its RNA into the cytoplasm. The difference is that included in this package is an enzyme known as reverse transcriptase. Once inside the cell, the reverse transcriptase goes to work, and starts copying the RNA into DNA. Another enzyme, viral integrase, brings this new DNA into the nucleus, where it is merged to the host’s DNA. This means two things: first, since these new genes are now part of the host’s DNA, mRNA will make a complementary copy of the codons, and bring them back out into the cytoplasm to be converted into proteins. Secondly, the headcrab’s genetic material is now permanently a part of the host’s DNA, and when the cell divides, the copied genes are brought along into the new cell. This is the most likely theory, and it would account for the radical, rapid, and widespread changes made throughout the body. It’s also a much more sophisticated and surefire way of making sure the information is copied, and can be recopied over and over again throughout the cells.

Finally, this headcrab field guide answers how a human/headcrab hybrid could take so much damage and be so aggressive:

It’s a safe bet that headcrabs are, in some way, controlling the hormones flowing through their hosts’ bodies…A few of the things the endocrine system controls are bone growth, useful for elongating human fingers into claws; metabolism, speeding it up or slowing it down to conserve energy; endorphins, the body’s natural painkillers; and adrenaline.

The Half-Life headcrab, if it really existed, would certainly be one of the most sophisticated parasites ever discovered. The manipulation of its host’s body and mind is unprecedented. That’s not to say that there aren’t real world analogs to a zombie-making, body-consuming parasite. In fact, nature is loaded with them.

Sacculina is a kind of barnacle that seeks out and castrates crabs. This yellowish blob of a creature attaches itself to the underside of some unfortunate crab and then inserts a part of its own body inside, harvesting nutrients and preventing the crab from laying any eggs. It goes further than that. Sacculina then releases hormones that chemically change the crab, no matter its sex, into something more like a female crab. And the parasite isn’t done there. After two sacculina mate on the underside of this poor crab, the hijacked crab then stirs the fertilized sacculina eggs in the ocean with its claw as though it were its own. The takeover is total.

If you are a cockroach, you never, ever want to meet an emerald jewel wasp. When one of these parasitic wasps finds a cockroach, it uses a highly adapted stinger to perform a very specific brain surgery. It injects a cocktail of chemicals into the roach’s brain (in an incredibly specific place in the brain, like a good surgeon) and the cockroach is powerless to stop it. After the surgery, the wasp bites down on one of the roach’s antennae, sucking nutrients from it and leading the roach like a dog on a leash down into the wasp’s burrow. The roach doesn’t try to get away either, the brain surgery has changed its nature. Once inside the burrow, it’s the usual fare of the wasp putting a baby in the roach to eat it alive from the inside out.

Sacculina and the emerald jewel wasp aren’t even the best cases of turning animals into zombies. A species of the cordyceps fungus is a parasite that exclusively turns ants into mindless spore factories. After the fungus has infiltrated the ant, the ant begins to act weird. It stops foraging for food and following the orders from the hive. Instead, it climbs up a nearby branch and uses its jaws to death-grip the underside of a leaf. It stays there, locked in place, until the fungus bursts through the tops of the ant’s head, raining down more spores on more unfortunate ants.

Cordyceps is so famous for making what are basically real zombies that a recent PS3 masterpiece—The Last of US—used the fungus to destroy humanity in a believable way.

But what about us? Nature has subverted the minds of ants and cockroaches and crabs but not us…right?

Toxoplasmosis gondii is a parasite that changes the behavior of rats. For this parasite to complete its life cycle, it must jump from the body of a rat into the stomach of a cat. To do this, it forms cysts that permanently alter the rat’s behavior — the rats no longer fear the smell of cats. This brash change of behavior makes them more likely to be eaten, and therefore more likely to help toxoplasmosis complete its life cycle.

But recent studies have shown that the presence of toxoplasmosis in humans is correlated with higher rates of schizophrenia and even risk of suicide. The talk below from science journalist extraordinaire Ed Yong is a great primer on the subject:

Could we be being manipulated to the extent that Half-Life’s headcrab imagines? Maybe. Better keep that crowbar handy.

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IMAGES:

The Secret Life of Headcrabs created by ajhateley

Sacculina carcini by Hans Hillewaert / CC-BY-SA-3.0

Ampulex compressa by Muhammad Mahdi Karim

Ant infected with Cordyceps by Cyndy Sims Parr

Toxoplasmosis by Nephron