Microbiome Metabolome Brain Vagus Nerve Circuit in Disease and Recovery focuses on the emerging hypothesis of a dysfunctional microbiome metabolome vagus nerve brain circuit in Alzheimer’s disease and associated diseases and medical conditions, including dementia, aging, COVID-19, autoimmune conditions, and inflammatory skin condition rosacea, which may increase the risk of other conditions. This book also discusses the vagus nerve-related conditions, including Arnold’s reflex, laryngopharyngeal reflux, duodenogastric reflux, gastroesophageal reflux, and related pulmonary diseases. The subjects covered in the book also address an important question of which one is more important for human health and intellectual abilities: the human genome or the human microbiome? The conceptual model of food and gut microbial tryptamine vagus nerve circuit is also presented in this book.