Risk factors for disease aren’t always accompanied by obvious symptoms. For instance, you can have high blood pressure or high cholesterol without obvious signs of heart disease.
The gut microbiome operates similarly. You can have massive problems among that community of organisms living in a symbiotic relationship within you and not have any obvious signs of disease. And yet, problems in the gut microbiome should serve as an alarm for autoimmune diseases, which affect 1 in every 5 Americans. Some physicians use stool tests that measure different gut flora as biomarkers for disease.