A lot of people think of viruses and bacteria in our bodies as nothing more than pests. It’s certainly true that a lot of them do an excellent job of making us ill. But some viruses and bacteria merged with our ancestors over the course of billions of years, and if you were to have them removed from your body today, you’d die faster than if you’d gotten a massive dose of Ebola.

In order to breathe, we depend on sausage-shaped blobs in our cells called mitochondria. When I say we, I mean not just humans or animals, but a vast group of species known as eukaryotes, which also includes plants, fungi, algae, slime molds, and various amoeba-like creatures. Mitochondria use