4. Toxoplasma Gondii

The Toxoplasma Gondii is a tiny, single-celled parasitic organism known for its terrifying ability to control its host’s mind. As it can only reproduce in cats’ intestines, the parasite infects mice and makes them more adventurous and less fearful of cats — in fact, infected mice even seem to become drawn to cats. This sinister effect increases the rodent’s likelihood of being eaten and providing a chance for the parasite to get into a cat’s intestine and reproduce.