Neonatal necrotising enterocolitis (NEC) mostly affects the small premature infant in a neonatal intensive care unit (NICU). It is proposed that, because of physical isolation, the cleanliness of nursing procedures, and, in some cases, antibiotic treatment, the normal bacterial colonisation of infants in NICUs may be delayed. In such babies, colonised with one or a few species, the organisms multiply in the gut, unhindered by competitors. The immature gut takes up macromolecules intact, especially in the lower ileum, and toxic products from the growing bacteria may be absorbed and cause mucosal damage, initiating NEC. Thus, NEC may be a result of the NICU environment.