Brain Parasites
What are they?
Parasites that turn victims into mindless, zombie-like slaves are fairly common in nature. There's one called toxoplasmosa gondii that seems to devote its entire existence to being terrifying.
This bug infects rats, but can only breed inside the intestines of a cat. The parasite knows it needs to get the rat inside the cat (yes, we realize this sounds like the beginning of the most fucked-up Dr. Seuss poem ever) so the parasite takes over the rat's freaking brain, and intentionally makes it scurry toward where the cats hang out. The rat is being programmed to get itself eaten, and it doesn't even know.
Of course, those are just rats, right?