**Abstract**

Research on the probiotic effect in preventing or treating diseases has attracted scientists' attention for many decades. Findings of probiotics effects on human health indicate that they are not only no detrimental but also may have a beneficial effect on the host. Indeed, the effectiveness of probiotics depends on the type of utilized strain, duration, dose administration, and single or combined strains used that can be different in a specific disease. Therefore, probiotics can play a significant role in the treatment and prevention of different diseases through several mechanisms; for instance, stimulating respiratory immunity in the airway and enhancing resistance to respiratory tract infections, can prevent or reduce the duration of respiratory system diseases. By ameliorating glucose metabolism, reducing inflammation and oxidative stress in pancreatic cells, and preventing the destruction of β-pancreatic cells, may prevent the onset of diabetes and the pathogenesis of diabetic retinopathy as well. Moreover, using their metabolites, especially short-chain fatty acids production, probiotics may have an important effect on weight modifications. Finally, from the regulation of important neurotransmitters and regulation of inflammatory markers, it may be effective in mental disorders improvement.

**Keywords:** probiotics, microbiota, diarrhea, irritable bowel syndrome, inflammatory bowel disease, Crohn's disease, ulcerative colitis, respiratory diseases, metabolic disorders, mental disorders, diabetes, obesity
