Final answer:
The parasite Toxoplasma gondii causes rats to lose their fear of cats, facilitating its life cycle. It poses a risk of toxoplasmosis in humans.
Step-by-step explanation:
The parasite that alters the behavior of rats in such a way that they seek out cats is Toxoplasma gondii. This protozoan creature is particularly insidious because it needs to enter the gut of a cat to reproduce. Infected rats lose their instinctive fear of cat urine, increasing the likelihood that they will be preyed upon by cats. This manipulation enhances the transmission of the parasite to the definitive feline host where it can complete its life cycle. Human infection can occur through contact with cat feces or consuming undercooked infected meat, leading to a condition known as toxoplasmosis, which is especially dangerous for pregnant women and immunocompromised individuals.