Professor Joël Doré, Head of Research at the Unit for the Ecology and Physiology of the Digestive System at the French National Institute for Agricultural Research (Institut National de la Recherche Agronomique – INRA), already explained why the term ‘microflora’ or ‘gut flora’ has been replaced by ‘gut microbiota’. This time, Prof. Doré focuses on the composition of gut microbiota, discussing questions such as “what kind of organisms or life forms do we find there?” and recovers the idea that not all kinds of bacteria are pathogenic. The French scientist also discusses the role of the viruses that can be found in the human gut ecosystem and classifies them according to the most frequent, the ones that act against bacteria and the ones that come from plants.