**1. Introduction**

The word probiotic comes from the Latin word *pro* and the Greek word bios, which when joined together, literally means "for life." The concept of probiotics has evolved over the many millennia but modern scientific theory of it only began in the twentieth century. The term probiotics was first used in 1953 by Werner Kollath, a German scientist who defined it as an "active substance that is essential for a healthy development of life." The definition of the term changed multiple times through the century but the most accepted one comes from the Food and Agriculture Organization of the United Nations and the World Health Organization (FAO/WHO). They define probiotics as "live microorganisms which when administered in adequate amounts confer a health benefit on the host [1–3]."

Although probiotics are believed to confer important health benefits, including amelioration of C. diff colitis, inflammatory bowel disease, metabolic syndrome, etc., the understanding of the mechanisms of action of probiotics is limited. Thus, the aim of the present chapter is to review the immune modulatory effects of probiotics and

how it interacts with the host gut microbiome. We will also highlight the practical clinical uses of probiotics on human health and disease. Lastly, we will speculate on the future direction on the use of probiotics.
