There is increasing evidence that certain probiotic strains can be useful in improving human health. The use of probiotics has received attention as a natural way of restoring body's normal microbiota, and an alternative and inexpensive way of preventing or treating infectious diseases without side effects. The best-documented clinical application of probiotics comes from trials on the treatment of gastrointestinal infections, mainly infectious diarrhoea. The enhancement of local as well as systemic immune responses by probiotics also offers new opportunities for probiotics in preventing infections at distal mucosal surfaces, such as those in the oral cavity, respiratory and urogenital tracts. The underlying mechanisms of probiotics are still unclear, but may include strengthening of the non-immunological gut barrier, interference with pathogen adhesion and growth inhibition, and the enhancement of the local mucosal immune system in the gut, as well as of the systemic immune response.