Probiotics are bacteria, but sometimes fungi, which when taken by the oral route may give some health benefits. The most compelling evidence for beneficial effects of probiotics is in the prevention and reduction in the duration of symptoms related to gut infectious disease. There is also evidence to show that some specific probiotics are beneficial in Clostridium difficile diarrhoea in the elderly. As further and better controlled clinical studies have appeared, some specific probiotics also appear to have beneficial effects in perhaps preventing and reducing the duration of symptoms due to acquired upper respiratory tract infections. In an attempt to explain these effects, attention has turned to the effects of some specific probiotics on the immune system. There is evidence that some specific probiotics can alter monocyte and natural killer cell function in the blood. Evidence is also accumulating that taking some specific probiotics can boost antibody responses to oral and systemically administered vaccines. The effect when shown is modest and is not always seen in different studies to all vaccines, but there is enough of a trend to make the area worthy of further investigation, particularly to tease out the mechanisms involved.