The Intestinal Innate Immune System in Newborns. Development and Inflammation in Health and Disease

The goal of this observational study is to determine the normal development of the human intestinal immune system in premature and mature neonatal life and to determine the pathophysiology behind life-threatening gastrointestinal diseases that appear during early life. The main questions aim to answer are: * to determine the normal development of the human intestinal immune system in premature and mature neonatal life and to determine the pathophysiology behind life-threatening gastrointestinal diseases that appear during early life. * is to investigate the development of the immune system in relation to enteral nutrition during the neonatal period. Participants will be asked to give faecal samples from day 1 of life and weekly for the following weeks until discharge (preterm infants). Further, surgery faecal samples and intestinal tissue will be collected proximal and distal to the pathology. In cases with a stoma, and when the child will undergo later reversal surgery, tissue samples from the proximal and distal ends of the intestine will be collected together with fecal samples (preterm and children up to 1 year of age who need to undergo intestinal surgery due to atresia).