Mammals and birds can also be infected. But cats in particular play a crucial part in the life cycle of the parasite: When a cat eats an infected animal, Toxoplasma gondii ends up in its gut. It reproduces there, generating offspring called oocysts that are shed in the cat’s feces. The oocysts can last for months in the environment, where they can be taken up by new hosts.
In the 1990s, scientists discovered that mice and rats infected with Toxoplasma gondii lose their natural fear of cat odors — and in some cases even appear to become attracted to them. It was possible, researchers speculated, that the parasite had evolved an ability to influence the behavior of its rodent hosts, to raise the chances they might be eaten by cats.
Subsequent studies have shown that the parasite can change the wiring of fear-related regions of the rat brain.