Chapter 4

TOXOPLASMA

Flip a coin. Tails? Relax.

Heads? You've got parasites in your brain.

That's right. Half of us carry the Toxoplasma gondii parasite. But don't reach for the power drill just yet.

Toxoplasma is microscopic. Human immune systems usually kick its ass, so if you've got it, you'll probably never even know. In fact, toxoplasma doesn't even want to be in your head. Trapped inside your thick skull, under assault by your immune defenses, it can't lay eggs, which is a big evolutionary Game Over.

Toxoplasma would much rather live in your cat's digestive system, eating cat food and laying eggs. Then, when kitty takes a crap, these eggs wind up on the ground, waiting for other scurrying creatures. Like, say, rats.

A quick word about rats: They're basically parasite subway trains, carrying them from place to place. In my job, we call this being a vector. Rats go everywhere in the world, and they breed like crazy. Catching a ride on the Rat Express is a major way that diseases have evolved to spread themselves.

When toxoplasma gets into a rat, the parasite starts to make changes to its host's brain. If a normal rat bumps into something that smells like a cat, it freaks out and runs away. But toxoplasma-infected rats actually like the smell of cats. Kitty pee makes them curious. They'll explore for hours trying to find the source.

Which is a cat. Which eats them.

And that makes toxoplasma happy, because toxoplasma really, really wants to live in the stomach of a cat.

Parasite geeks have a phrase for what cats are to toxoplasma: the "final host."

A final host is the place where a parasite can live happily ever after, getting free food, having lots of babies. Most parasites live in more than one kind of animal, but they're all trying to reach their final host, the ultimate vector ... parasite heaven.

Toxoplasma uses mind control to get to its heaven. It makes the rat want to go looking for cats and get eaten. Spooky, huh?

But nothing like that would work on us humans, right?

Well, maybe. No one's really sure what toxoplasma does to human beings. But when researchers collected a bunch of people with and without toxoplasma, gave them personality tests, observed their habits, and interviewed their friends, this is what they learned:

Men infected with toxoplasma don't shave every day, don't wear ties too much, and don't like following social rules. Women infected with toxoplasma like to spend money on clothes, and they tend to have lots of friends. The rest of us think they're more attractive than non-infected women. In general, the researchers found that infected people are more interesting to be around.

On the other hand, people without toxoplasma in their brains like following the rules. If you lend them money, they're more likely to pay you back. They show up for work on time. The men get into fewer fights. The women have fewer boyfriends.

Could this be toxoplasma mind control?

But that's just too weird, isn't it? There has to be another explanation.

Maybe some people always hated getting to work on time, and they like having felines around because cats wouldn't go to work on time either, if they had any work to go to. These people adopted cats and then got toxoplasma.

Maybe the other half of humanity, the ones who enjoy following the rules, usually get dogs. Fetch. Sit. Stay. So, their brains stay toxoplasma-free.

Maybe these are two kinds of people who were already different.

Or maybe not.

That's the thing about parasites: It's hard to tell if they're the chicken or the egg. Maybe we're all really robots, walking around doing the bidding of our parasites. Just like those hungry, twitching snails ...

Do you really love your cat? Or is the toxoplasma in your head telling you to take care of kitty, its final host, so that one day it too will reach parasite heaven?