CAT AND MOUSE A parasite that lives to change mouse behavior may also be altering the way humans act. The parasitic protozoa Toxoplasma gondii thrives by cycling through feline and rodent hosts. When it infects mice, the brain-dwelling parasite makes them more daring and, in particular, less afraid of the scent of cats (so it can get passed back to the feline hosts when they eat the infected, emboldened rodents)... ISTOCKPHOTO/SHIFFTI