The human gastrointestinal tract is thought to have been colonized by more than 10 14 bacteria. Many bacterial species have adapted to the conditions of the human intestinal microflora, which is a rich source of nutrients. The conditions present in the intestines directly affect the appearance and development of certain microbial species. Humans and their intestinal microflora have evolved together over time and thus it is not surprising that their symbiotic relationship provides mutual benefits. Some factors such as geographical origin, age, genetics, type of birth, breastfeeding, stress, smoking, diet, and use of drugs and probiotics can significantly affect the abundance and diversity of intestinal microflora. Diet and lifestyle are among the key environmental factors of the intestinal microflora. Numerous findings to date indicate an association between intestinal microflora and various diseases in humans, including metabolic, immunological, and neurological diseases. Many diseases in humans are associated with dysbiosis of the intestinal microflora. This points us to the most important role of the intestinal microflora in human health, and that is that it connectsall parts of the body into a complete and organized biological system. It is important to point out that the association between intestinal microflora dysbiosis and some diseases can be both causal and consequential. In the treatment of certain diseases, potential dietary strategies are slowly being introduced, which change the composition of the intestinal microflora (probiotics and prebiotics). Further development of advanced 16S ribosomal gene sequencing of RNA and other techniques will provide even better insight into the microorganisms that inhabit our intestines and their effects on human health.