It's time to revisit that grand old parasite, the brain-infecting Toxoplasma. The more we learn about it, the more marvelously creepy it gets. Toxoplasma is a single-celled relative of the parasites that cause malaria. It poses a serious risk to people with compromised immune systems (for example, people with AIDS) and fetuses (which is why pregnant women need to avoid getting Toxoplasma infections). If you've got a healthy immune system, it doesn't cause any immediate harm. (Ed Yong has explained why a purported link to brain cancer is very weak.) All told, perhaps a quarter or a third of all people on Earth carry thousands of Toxoplasma cysts in their heads. Most never become aware of their living cargo. The Toxoplasma life cycle normally takes the parasite from cats to the prey of cats and back again. In the guts of cats, the parasites have sex and produce egg-like offspring which are shed with cat droppings. They can survive in soil for weeks or months. Rats and other mammals ingest the eggs, which produce cysts mainly in the brain. When the cats eat infected prey, they get infected. For a little over ten years, scientists have been investigating whether Toxoplasma raises its odds of getting back into cats by manipulating their prey hosts. Oxford researchers kicked thing off by releasing healthy and infected rats into large enclosures. They spritzed corners of the enclosure with various odors, including the urine of rats, rabbits, and cats. Normally rats become anxious the instant they sniff cat urine and explore much less. Wise move. Not so wise is the response of infected rats: in the enclosure experiments they either became indifferent to the smell of cats, or spent some extra time checking out the feline corner. There was no difference in how the infected rats responded to other smells. Robert Sapolsky, a neuroscientist at Stanford University, and his colleagues have carried the experimental torch foreward. In 2006, they demonstrated just how precise Toxoplasma's effects are. They found that infected rats did not lose their fear across the board. Dog urine still spooked them, and they could be trained to get scared of new stimuli. Only their innate fear of cats changed. Sapolsky's team then looked at where the parasite actually ended up in the rat brain. They foundToxoplasma cysts clumped around the amygdala, a region of the brain that's heavily involved in fear and other emotions. Now Sapolsky and his colleagues have looked even closer at the parasite's effects. They had rats sniff various odors and then examined their brains to look for a telltale protein called c-Fos. When neurons fire, they produce c-Fos, and so the more active a region of the brain, the more c-Fos accumulates in it. The scientists found two big differences in infected rat brains when they sniffed cat urine, both of which occurred in the region around the amygadala. A circuit in the brain that helps produce defensive behaviors became less active. Near that circuit is another circuit that triggers sexual arousal. And the parasite also altered this sexual arousal circuit. It increased the activity of those neurons. Really, it would have been mind-blowing enough for a parasite to surgically swoop into a host brain and knock out the fear it felt towards a particular animal. We admirers of our parasite overlords would have been satisfied. But the possibility that these hosts are actually attracted to their enemy, that they feel the deepest desire a rat can feel, a desire that could lead them to death, and lead the parasite to live on, to achieve their own deepest sexual desires--well, we can only be grateful.