After delivery, plastic wraps or warm mattresses are useful to keep the infant warm on their way to the neonatal intensive care unit (NICU).[128][needs update] In developed countries premature infants are usually cared for in an NICU. The physicians who specialize in the care of very sick or premature babies are known as neonatologists. In the NICU, premature babies are kept under radiant warmers or in incubators (also called isolettes), which are bassinets enclosed in plastic with climate control equipment designed to keep them warm and limit their exposure to germs.