What is it about the microbes in our guts that can have such an impact on our lives? The human gut has around 100 trillion bacterial cells from up to 1,000 different species. Every person's microbiota (the body's bacterial make-up) is different as a result of the effects of diet and lifestyle, and the childhood source of bacteria. Scientists are learning more and more about the importance of these bacteria, as well as the viruses, fungi and other microbes that live in our gastrointestinal tracts. Without them, our digestion, immune system and overall health would be compromised. Adam Hart talks to researchers who are discovering how important a balanced and robust gut microflora is for our health. And he asks how this can be maintained and what happens when things go wrong.