This book unravels in an easily understandable way the exciting molecular connections between our gut microbiome, i.e. the collective microorganisms that we carry in our digestive system, nutrition and the immune system, and how an imbalance can lead to the development of inflammatory diseases. The totality of these gut microorganisms forms a complex ecosystem within us, consisting of hundreds of different species of bacteria. The author, microbiologist Professor Richard Lucius, introduces the historical context of microbiome research, discusses how our lives have changed in recent decades, and explains the composition and function of the microbiome and its connection to the gut and immune system. The book explains in detail and supported with appealing illustrations how allergies, chronic intestinal inflammation, type 1 diabetes, multiple sclerosis, rheumatoid arthritis, psoriasis and coeliac disease develop. Further chapters deal with the effects of nutrition on our intestinal flora and provide tips on building and maintaining a healthy microbiome. An intact bacterial community utilizes food residues that humans cannot digest and in return provides energy and vital substances. Only together with our inhabitants do we form a well-functioning "superorganism". Our immune system, which has developed over millions of years, is calibrated for a meagre, natural life. Under the influence of our modern way of life, the "Western lifestyle", our inner ecosystem is suffering from a loss of species, similar to our external environment. Highly processed foods, the influence of medication, lack of exercise and little contact with environmental bacteria are promoting this erosion process, which in the long term increases the risk of inflammatory diseases and obesity. As a result, more than 25% of people in modern societies suffer from allergies, autoimmune diseases and food intolerances that hardly existed in the past. This development can be counteracted by a healthy lifestyle that strengthens and diversifies the microbiome. This book is aimed at anyone who wants to understand the complex interplay between our gut microbiome, the immune system, nutrition and disease. The English translation of this volume, originally published in German language, was facilitated by artificial intelligence. A language editor has subsequently revised the text further in an endeavor to refine the work stylistically. The content was later revised by the author for accuracy.