Toxoplasmosa gondii is a brain parasite that infests the brains of rats but reproduces in the intestines of cats. The interesting part is that the parasite
takes over the rats brain, and the rat unknowingly allows itself to be eaten by the
cat.

In a nutshell-
step 1) toxoplasmosa gondii takes over rats brain (as in mind control)
step 2) toxoplasmosa gondii must reproduce in the intestines of a cat to complete its life cycle
step 3) toxoplasmosa gondii forces the rat to be submissively eaten by the cat, whearpon it will shed its eggs to be picked up by other rats and animals.
terrifying semi-step 4) toxoplasmosa gondii exists in the brains of over half the worlds population (over three billion people)
terrifying semi-step 5) human and rat brains share the same basic anatomy and neurotransmitterrs, which is why rats are common in human drug expirements
terrifying semi-step 6) the possibility of viral mutation brings the possibility that the virus could adapt to the human anatomy and have us behave in the same way as the rats, bringing a loss of self preservation and willpower
even more terrifying semi-step 7) with the modern trend of biological weaponization that several countries have poured money into (see tularemia, anthrax, mustard gas, etc), it is a matter of time before viruses such as toxoplasmosa gondii or exactly like toxoplasmosa gondii are weaponized, or at least mutated (see three mile island
wildlife).
even more horrifying step 8) possibility of near zombie-like apocalypse
not to mention cases of humans carrying the toxoplasmosa gondii virus often exhibit signs of skitzophrenia and general unpleasantness.