Once the parasite has reached the mosquito’s mouth, though, it wants the mosquito to start biting as much as possible. Plasmodium travels to the salivary glands, homing in on a lobe that is responsible for making the anticoagulant molecule apyrase. There it proceeds to cut off the mosquito’s apyrase supply, so that when the insect drives its proboscis into a new host, it has a harder time keeping the blood flowing. It has to visit more hosts to drink the same amount of blood. At the same time, Plasmodium makes the mosquito hungrier, drinking more blood and visiting more hosts to get it. As a result, a sick mosquito is twice as likely as a healthy one to drink the blood of two people in a night. The sick mosquito, carrying more blood to more hosts, becomes a far more effective way to spread malaria.
Plasmodium makes a predator—a mosquito—come into contact with its prey—us. Parasites can use the opposite arrangement as well, by living first in prey and waiting until a predator eats it. Some parasites are willing to sit and wait for their intermediate host to be devoured. But many are not so patient. A fluke called Leucochloridium paradoxum makes snails its first host, but makes insect-eating birds its final host, even though the birds have no appetite for snails. The flukes get the bird’s attention by pushing their way into the eye tentacles of the snail. Covered in brown or green stripes, the parasites are visible through the transparent tentacles, and to a bird they look like caterpillars. A bird attacks the snail and ends up with nothing but a bellyful of parasites.
Other parasites can change their host’s skin to become a more obvious target. Some species of tapeworms live in the guts of the threespine stickleback fish for a few weeks, and when they want to get into a bird, they turn the fish orange or white. They can also alter the behavior of the fish to get the attention of the birds. Normally, sticklebacks keep diligently away from the water birds that like to eat them. They try to stay well below the water’s surface, and if a heron should stick its head underwater, they will dart away, passing up the opportunity to eat. But when they are infected by tapeworms, they become buoyant so that they can’t help but swim near the surface, and they become fearless as well, chasing after food even if a bird is dangerously close by.
Sometimes it’s not enough for a parasite to make its host vulnerable to attack; sometimes it sends its host straight into harm’s path. Such is the case with thorny-headed worms. Many species of these parasites start off inside invertebrates that live in lakes and rivers. They then become adults in birds, where they drive their barbed heads deep into the lining of the intestines. A small crustacean named Gammarus lacustris feeds near the surface of ponds and rivers, but as soon as its predator—a duck—comes around, it escapes by diving away from the light and thus down to the bottom of the water. When a thorny-headed worm gets inside a Gammarus, though, it does the exact opposite. If a duck comes on the scene, Gammarus feels an unshakable attraction toward light—and thus moves up to the surface of the water. When it reaches the surface, it skims along until it finds a rock or a plant. Once it makes contact, it clamps its mouth down, practically offering itself up to the duck.
Toxoplasma, the protozoan lodged in billions of human brains, may seem like a gentle creature that wouldn’t get involved in mind control. After all, it hides safely in its cysts and declines to kill its hosts. But its tameness is only part of its unconscious calculation of how to boost its odds of getting into its final host. Toxoplasma needs to move between cats and their prey and back to complete its life cycle, and a dead rat won’t attract many cats. But Toxoplasma, it turns out, does what it can to help the cats kill their prey.
For several years scientists at Oxford University have been studying the effects of Toxoplasma on the behavior of rats. They built a six-foot by six-foot outdoor enclosure and used bricks to turn it into a maze of paths and cells. In each corner of the enclosure they put a nest box along with a bowl of food and water. On each nest they added a few drops of a particular odor. On one they added the scent of fresh straw bedding, on another the bedding from a rat’s nest, on another the scent of rabbit urine, on another the urine of a cat. When they set healthy rats loose in the enclosure, the animals rooted around curiously and investigated the nests. But when they came across the cat odor, they shied away and never returned to that corner. This was no surprise: the odor of a cat triggers a sudden shift in the chemistry of rat brains that brings on intense anxiety. (When researchers test anti-anxiety drugs on rats, they use a whiff of cat urine to make them panic.) The anxiety attack made the healthy rats shy away from the odor and in general made them leery of investigating new things. Better to lie low and stay alive.
Then the researchers put Toxoplasma-carrying rats in the enclosure. Rats carrying the parasite are for the most part indistinguishable from healthy ones. They can compete for mates just as well and have no trouble feeding themselves. The only difference, the researchers found, is that they are more likely to get themselves killed. The scent of a cat in the enclosure didn’t make them anxious, and they went about their business as if nothing was bothering them. They would explore around the odor at least as often as they did anywhere else in the enclosure. In some cases, they even took a special interest in the spot and came back to it over and over again.
By turning rats into rodent kamikazes, Toxoplasma probably increases its chances of getting into cats. If it makes the mistake of getting into a human instead of a rat, it has little hope of making that journey, but there’s some evidence that it still tries to manipulate its host. Psychologists have found that Toxoplasma changes the personality of its human hosts, bringing different shifts to men and women. Men become less willing to submit to the moral standards of a community, less worried about being punished for breaking society’s rules, more distrustful of other people. Women become more outgoing and warmhearted. Both changes seem to break down the fear that might keep a host out of danger. They’re hardly enough to make people throw themselves at lions, but they’re a very personal reminder of the ways in which parasites try to take control of their destiny.
Scientists have known about these sorts of transformations for more than seventy years, but they didn’t think they were actually manipulations. Parasites couldn’t possibly mastermind pinpoint changes to their plainly superior hosts. They could only cause random kinds of harm, and maybe by chance the damage altered their host. Only in the 1960s did scientists begin to think seriously about the possibility that a parasite might be able to engineer the physiology of its host, or even its behavior. And thereupon emerged a long line of cases that seemed, on their faces, to be just that.
Most of the cases came from eukaryote parasites, although certainly bacteria and viruses can act as puppet-masters sometimes. A sneeze carries away cold viruses to new hosts; the Ebola virus seems to take advantage of our respect for the dying and the dead by making its victims gush blood, which gets on the bodies of people handling their bodies, infecting them as well. But if you look over the documented cases of manipulators, bacteria and viruses make up a tiny portion. It may be that their needs are pretty simple: they rarely need to use more than one species as a host, and they can just ride along during the regular contacts between hosts—be it sex, a handshake, or the bite of a tick. There may in fact be a lot of manipulators waiting to be revealed among bacteria and viruses. They may still be hidden, thanks to the fact that most people who study viruses and bacteria primarily think in terms of diseases, symptoms, and cures. They tend not to think like parasitologists, who treat their subjects more as living beings that have to survive in their hosts and get to new ones.