Don't freak out, but the human body — and probably your body — is swarming with tiny parasites. If we were to count every cell in your body, only about 43 percent would be human. The rest are bacteria, viruses, parasites and other single-celled organisms. Not all of them are bad guys — some can even be beneficial for your health. But one hitchhiking microbe called Toxoplasma gondii really gets around, with around one-third of the planet encountering this tiny pathogen in their lifetimes.
T. gondii isn't a virus or a bacteria. It's a protozoan, similar to the malaria parasite. In most people, T. gondii doesn't cause any problems. However, in other mammals, especially rodents, it can change the behavior of its host, causing it to approach predators like cats. The felines appreciate an easy meal and the T. gondii appreciates being able to breed in the cats' guts, spreading through its feces and repeating its life cycle. It's a real world example of zombies, which is why this tiny bug has long fascinated people.
