A probiotic is a product containing live microorganisms which, when consumed in adequate amounts, confer a health benefit on the host. The classic vehicles are fermented dairy: yoghurts and drinks carrying Lactobacillus and Bifidobacterium strains, with kefir, kimchi, and sauerkraut as traditional food sources. Documented benefits include shortening certain diarrhoeal illnesses, easing irritable bowel symptoms in some patients, and supporting gut barrier function, though effects are strain-specific and a benefit shown for one strain cannot be claimed for another. Regulatory control is strict in the EU, where the very word “probiotic” on labels implies a health claim requiring authorisation, and looser elsewhere. Quality varies: products must deliver viable organisms through shelf life and stomach acid, which is why reputable brands specify strains and colony-forming units.