We have heard the adages often of "the gut feeling" or "feeling sick in the stomach" or "butterflies in the stomach". Often when we are ill, the appetite is often a symptom. This makes our gut a vital indicator of health. Along with the genetic material we inherit the gut microbiota. With this background, research is uncovering alterations in our gut microbiota in several diseases. Apart from well-known inflammatory conditions such as irritable bowel syndrome, this book looks at how our gut tenants are altered and the consequences in other conditions like nervous system diseases, diabetes, cancer, lung diseases, the latest COVID-19 epidemic, and depression.