Neonatal infections are infections that occur in the neonate (new born) during prenatal development or the first four weeks of life (neonatal period). Neonatal infections can be acquired by mother-to-child transmission, in the birth canal during childbirth, or after birth. Some neonatal infections appear immediately after birth, whereas others may appear later in life. Neonatal intensive care has evolved substantially in high-income countries during the previous few decades. In these situations, neonatal infections have a considerable morbidity and mortality burden in the extremely preterm population. When compared to other age groups, the immaturity of the immune system in the neonatal era, particularly in preterm new-borns, confers specific clinical, physical, and outcome characteristics to infections: Neonates are more sensitive to a wide variety of infections.