The gut is home to millions of microscopic organisms, which together are called the gut microbiota. Most of these microorganisms live in peace with the rest of our cells, help us get energy from food, and give us essential nutrients that we cannot make ourselves. The cells lining the gut’s surface keep these microorganisms separated from the rest of the body, like the “Great Wall” of our kingdom. On the inside of this Great Wall, the heroes of the mighty immune system watch over and protect us. But how can the immune system tell the difference between our own cells and the gut microorganisms? Or between helpful microorganisms and disease-causing ones? It is an amazing storey, full of secrets. In this article, we will explain how the gut coaches the immune system to recognise enemies, and how some aspects of modern life might be interfering with this process.