Lactose, the sugar in milk, and other foods not fully digested or absorbed in the small intestine are metabolized by bacteria and Archaea in the large intestine producing hydrogen, methane and metabolic toxins resulting in gut and systemic symptoms. Diagnosis of lactose and food intolerance requires identification of clinical symptoms, a DNA test, and measurement of breath hydrogen and methane. Similar symptoms are also caused by FODMAPS (fermentable oligosaccharides, disaccharides, monosaccharides, polyols) and Coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for the coronavirus disease 2019 (COVID-19) pandemic.