Chapter 12

THE MASTER PARASITE

Meet wolbachia, the parasite that wants to rule the world.

Wolbachia is tiny, smaller than a single cell, but its powers are enormous. It can change its hosts genetically, tamper with their unborn children, and create whole new species of carriers ... whatever it takes to fill the world with wolbachia.

No one knows how many creatures on earth are infected. At least twenty thousand insect species carry it, and so do a lot of worms and lice. That's trillions of carriers as far as we know. And every new place scientists look, they find more.

So, do you have to worry about wolbachia? We'll get back to that later.

Here's the strangest thing about wolbachia: No living creature has ever been infected with it. That's right, you don't catch wolbachia; you are born with it.

Huh?

You see, wolbachia is like one of those scrawny supervillains with a big brain. Wolbachia is a wimp: It can never leave its host's body, not even in a drop of blood. At some point in its evolutionary history, wolbachia gave up the whole jumping-between-creatures thing and adopted a strategy of staying in safe territory - it spends its whole life inside one host.

So how does it spread? Very cunningly. Rather than risk the outside world, wolbachia infects new carriers before they're born. That's right: Every infected creature gets the disease from its own mother.

But what happens when wolbachia is born into a male host? Males can't have children, so they're a dead end for the infection, right?

This is the evil genius part.

In many insect species, wolbachia scrambles its male hosts' genes with a secret code. Only other wolbachia (living inside a female) know how to decode the genes and make them work right. So when an infected insect tries to mate with a healthy one, the kids are born with horrible mutations, and they all die.

Over hundreds of generations of breeding only with one another, the infected insects slowly evolve into a new species. This species is one hundred percent infected with wolbachia and dependent on its own parasite to have children. (Insert evil laugh here.)

And this isn't wolbachia's only species-altering trick.

In some kinds of wasps, wolbachia has an even more power-crazed solution to the male problem. It simply changes all its host's unborn children into females. No boys ever get born. Then wolbachia gives these females a special power: They can have kids without mating. And of course, all of those kids are born female too. In other words, males become completely irrelevant. Because of wolbachia, some species of wasp have become entirely female. All the boys are dead.

In fact, some scientists believe that wolbachia's tricks may be responsible for creating a big chunk of the insect and worm species on our planet. Some of those species, like parasitic wasps, go on to infect other creatures. (That's right, even parasites have parasites. Isn't nature wonderful?) In this way, wolbachia is slowly remaking the world in its own image; without ever leaving the safety of home.

So what about you? You're not an insect or a worm. Why worry about wolbachia?

Meet the filarial worm, a parasite that infects biting flies. It happens to be one of wolbachia's big success stories. All of these worms are infected. If you "cure" a filarial worm with antibiotics, it can't have kids anymore. It's dependent on its own parasites - one of many species genetically engineered to be wolbachia carriers.

So what happens when a fly infected with filarial worms bites you? The worms crawl into your skin and lay eggs there. The eggs hatch, and the babies swim around in your bloodstream, some of them winding up in your eyeballs. Fortunately, the baby worms don't hurt your eyes. Unfortunately, the wolbachia they carry sets off a red alert in your immune system. Your own immune system attacks your eyeballs, and you go blind.

Why does wolbachia do this? What is its evolutionary strategy in blinding human beings?

No one knows. One thing is for sure, though: Wolbachia wants to rule the world.