Counterintuitive Facts (5): Your Anger Is a Parasite Reproducing in Someone Else's Brain

I. Mice fear cats. This is instinct carved into DNA by hundreds of millions of years of evolution. Hear a cat's meow, run. Smell cat urine, avoid. This is the only rule for survival.

II. But scientists discovered a group of mice that don't fear cats. Not only don't they fear cats, they're attracted to the smell of cat urine. They'll actively approach cats, even strut around in front of them. Then get eaten.

III. Have these mice gone mad? No. They're infected with a parasite called Toxoplasma gondii. This parasite can only reproduce in cat intestines. Mice are intermediate hosts. Cats are the ultimate host. To get from mouse back into a cat, the toxoplasma hijacked the mouse's brain. It modified the mouse's dopamine circuits, turning "fear" into "pleasure."