About half of your body consists of microbiota – bacteria, protists, viruses, and fungi! The microbiome is the community of these microorganisms living in a specific habitat (such as your guts!) plus their genetic material. In this educational tool-kit you will learn how the microorganisms can be crucial for immunity, hormones and even the behaviour of animals. We will use fish as our main study object. Because fish and many mammals like humans are not that different after all (we all came from a fishlike ancestor), you will learn about yourself too!