Probiotics are live microorganisms that occur naturally in the human digestive system. They chiefly colonise the colon and benefit the health of the individual by acting as a digestive aid and boosting the immune response. Low levels of beneficial bacteria and microorganisms in the colon provide a greater opportunity for pathogenic bacteria to become established. Probiotics may be beneficial in reducing the severity and incidence of gastroenteritis, diarrhoea, and colic in babies. They can also benefit children's health by supporting the developing immune system but should not be seen as a panacea or as an alternative to a healthy lifestyle or diet. The level of probiotics in the colon can be boosted by a healthy diet containing plenty of grains, fruit and vegetables. It can also be boosted by probiotic supplementation. Supplementation by the probiotic L. reuteri has been studied in major scientific trials and proven to be safe and effective with no reported side effects.