You may have read about the fungus that can infect an ant and turn it into a zombie, making it climb up the stem of a plant and hang onto it, then die and release spores from a stalk that grows out of its head.

But this isn’t the only parasite that controls the behavior of its host.

If you ever got sick, had diarrhea, and thought hard about why, you’ll understand what I mean. You were helping spread the disease… especially if you were poor and didn’t have a toilet. This is why improved sanitation actually reduces the virulence of some diseases: it’s no longer such a good strategy for bacteria to cause diarrhea, so they evolve away from it!

There are plenty of other examples. Lots of diseases make you sneeze or cough, spreading the germs to other people. The rabies virus drives dogs crazy and makes them want to bite. There’s a parasitic flatworm that makes ants want to climb to the top of a blade of grass, lock their jaws onto it and wait there until they get eaten by a sheep! But the protozoan Toxoplasma gondii is more mysterious.

It causes a disease called toxoplasmosis. You can get it from cats, you can get it from eating infected meat, and you can even inherit it from your mother.

Lots of people have it: somewhere between 1/3 and 1/2 of everyone in the world!

A while back, the Czech scientist Jaroslav Flegr did some experiments. He found that people who tested positive for this parasite have slower reaction times. But even more interestingly, he claims that men with the parasite are more introverted, suspicious, oblivious to other people’s opinions of them, and inclined to disregard rules… while infected women, are more outgoing, trusting, image-conscious, and rule-abiding than uninfected women!

What could explain this?

The disease is carried by both cats and mice. Cats catch it by eating mice. The disease causes behavior changes in mice: they seem to become more anxious and run around more. This may increase their chance of getting eaten by a cat and passing on the disease. But we are genetically similar to mice… so we too may become more anxious when we’re infected with this disease. And men and women may act differently when they’re anxious.

It’s just a theory so far. Nonetheless, I won’t be surprised to hear there are parasites that affect our behavior in subtle ways. I don’t know if viruses or bacteria are sophisticated enough to trigger changes in behavior more subtle than diarrhea… but there are always lots of bacteria in your body, about 10 times as many as actual human cells. Many of these belong to unidentified species. And as long as they don’t cause obvious pathologies, doctors have had little reason to study them.

As for viruses, don’t forget that about 8% of your DNA is made of viruses that once copied themselves into your ancestors’ genome. They’re called endogenous retroviruses, and I find them very spooky and fascinating. Once they get embedded in our DNA, they can’t always get back out: a lot of them are defective, containing deletions or nonsense mutations. But some may still be able to get back out. And there are hints that some are implicated in certain kinds of cancer and autoimmune disease.

Even more intriguingly, a 2004 study reported that antibodies to endogenous retroviruses were more common in people with schizophrenia! And the cerebrospinal fluid of people who’d recently gotten schizophrenia contained levels of a key enzyme used by retroviruses, reverse transcriptase, four times higher than control subjects.

So it’s possible—just possible—that some viruses, either free-living or built into our DNA, may change our behavior in subtle ways that increase their chance of spreading.

For more on Jaroslav Flegr’s research, read this fascinating article:

• Kathleen MacAuliffe, How your cat is making you crazy, The Atlantic, March 2012.

Among other things you’ll read about the parasitologists
Glenn McConkey and Joanne Webster, who have shown that Toxoplasma gondii has two genes that allow it to crank up production of the neurotransmitter dopamine in the host’s brain. It seems this makes rats feel pleasure when they smell a cat!

(Do you like cats? Hmm.)

Of course, in business and politics we see many examples of ‘parasites’ that hijack organizations and change these organizations’ behavior to benefit themselves. It’s not nice. But it’s natural.

So even if you aren’t a disease-spreading zombie, it’s quite possible you’re dealing with them on a regular basis.