Several clinical studies have demonstrated the therapeutic and/or prophylactic efficacy of specific probiotics against acute viral gastroenteritis and antibiotic-associated diarrhoea (including Clostridium difficile infection). Emerging evidence also suggests beneficial effects against Helicobacter pylori infection. The evidence of efficacy against traveller's diarrhoea remains, however, inconclusive. The precise mechanisms by which probiotics potentiate host gastrointestinal defences and mediate protection are not fully known. There is evidence to suggest, however, that probiotics might contribute to host defence by reinforcing non-immunological defences and stimulating both specific and non-specific host immune responses. Little is known about the relative importance of the probiotic-stimulated mechanisms in host protection. This review summarises the evidence for the anti-infective effects of probiotics and discusses the effect of orally delivered probiotics on non-immunological and immunological defence mechanisms in the host, especially in the gastrointestinal tract.