'Viable bacteria that exhibit beneficial effects for health based on improvement of balance of intestinal bacterial flora' was the most common definition of probiotics, but the diversity of their action mechanisms, including immune regulation, has been clarified, and probiotics have recently been broadly defined as 'live microorganisms which when administered in adequet amounts confer a health benefit on the host'. Coined words such as prebiotics, describing non-digestible food fiber components that contribute to host health by activating proliferation and function of beneficial intestinal bacteria, and synbiotics, describing a combination of probiotics and prebiotics have also been established as medical expressions. In this report, clinical studies of probiotics, prebiotics, and synbiotics on prevention of infection by various infectious diseases in humans are reviewed. The effects of and action mechanisms against sporadic intestinal infectious diseases that are difficult to investigate in humans (enterohemorrhagic Escherichia coli and Salmonellosis) in experimental animal models are also reviewed. Finally, points necessary for clarification of the role of probiotics in health care, and their functions in health care foods are discussed.