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 ...