It inhibits the fear response in rodents, so they are more likely to cuddle up with cats

The joke being that humans are infected too

In rodents, T. gondii alters behavior in ways that increase the rodents' chances of being preyed upon by felids. Support for this "manipulation hypothesis" stems from studies showing that T. gondii-infected rats have a decreased aversion to cat urine while infection in mice lowers general anxiety, increases explorative behaviors and increases a loss of aversion to predators in general. Because cats are one of the only hosts within which T. gondii can sexually reproduce, such behavioral manipulations are thought to be evolutionary adaptations that increase the parasite's reproductive success since rodents that do not avoid cat habitations will more likely become cat prey

Apparently it is a popular American myth.

@Caligvla as @otter said, T gondii stops rodents from being afraid of cats. It also makes male rodents sexually attracted to cats.