Pediatrics (also spelled paediatrics or pædiatrics) is the branch of medicine that deals with the medical care of infants, children, and adolescents, and the age limit usually ranges from birth up to 18 years of age (in some places until completion of secondary education, and until age 21 in the United States). A medical practitioner who specializes in this area is known as a pediatrician, or paediatrician. The word paediatrics and its cognates mean "healer of children"; they derive from two Greek words: παῖς (pais "child") and ἰατρός (iatros "doctor, healer"). Pediatricians work both in hospitals, particularly those working in its specialized subfields such as neonatology, and as primary care physicians who specialize in children.