There are trillions of tiny organisms living inside your gut right now—and they may be influencing your emotions more than you think. Section 1: What Is the Microbiome? Microbes are tiny living things that are usually too small to see without a microscope. They can be found everywhere on Earth and living inside our bodies. Main Types of Microbes Bacteria: Single-celled organisms with no nucleus that live almost everywhere. Viruses: Tiny infectious agents that need a host cell