At birth, all newborn infants have to adapt to the dramatic transition from a sterile life in the mother to an environment where environmental microbes colonize all epithelial surfaces of the body (e.g. gut, lungs, skin) within the first days and weeks after birth. Adaptation requires a well-developed innate immune system at epithelial surfaces, that allows tolerance to harmless or beneficial microbes and exclusion of harmful pathogens. After a normal gestation length, delivery process, maternal care and hygiene, newborn infants adapt well, but a markedly higher prevalence of infection, sepsis and inflammatory disorders in early life nevertheless shows that infants and children are very sensitive to the dramatic microbiological and immunological transition at birth.

Of the 15 million infants born preterm each year worldwide, more than 1 million die. Many of these deaths relates to their immature ability to fight bacterial colonization at the gut, lung and skin surfaces. Infections are related to immature lung, skin and gut barrier functions and especially tolerance to the great number of bacteria along the gut surface from a few days after birth (≈10 billion) is a problem. These infections induce impaired gut function and pathogens/toxins may translocate into the blood stream, create sepsis and inflammatory reactions throughout the body.

Morbidity is inversely related to gestational age, however there is no gestational age, including term that is wholly exempt. Although most organs are immature, the brain and lung are especially susceptible to the consequences of preterm birth, which leads to high rates of long-term neurological and health problems. In a review, more than half of ELBW children were readmitted to hospital at least once in the first 1-2 years of life, mostly as a result of respiratory illnesses. Most studies of very low birth weight (VLBW) infants show continued sequelae such as cognitive deficits, academic underachievement, grade failures, and the need for increased remedial assistance during mid-childhood and adolescence.