Pre- and probiotics live in our guts, where they help us to digest food. Not only are most gut bacteria harmless, but we couldn’t survive without them. However not all bacteria are equal, and a shift in the balance of 'good to bad' bacteria has been linked with a long list of medical conditions from obesity and heart disease to bowel problems and cancer.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/feeding-your-good-bacteria-with-probiotics-and-prebiotic-supplements