Pediatrics (also spelled paediatrics or pædiatrics) is the branch of medicine that involves the medical care of infants, children, and adolescents. The American Academy of Pediatrics recommends people be under pediatric care up to the age of 21. A medical doctor who specializes in this area is known as a pediatrician, or paediatrician. The word pediatrics 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 subspecialties such as neonatology, and as outpatient primary care physicians. Pediatrics (also spelled paediatrics or pædiatrics) is the branch of medicine that involves the medical care of infants, children, and adolescents. The American Academy of Pediatrics recommends people be under pediatric care up to the age of 21. A medical doctor who specializes in this area is known as a pediatrician, or paediatrician. The word pediatrics 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 subspecialties such as neonatology, and as outpatient primary care physicians. Already Hippocrates, Aristotle, Celsus, Soranus, and Galen understood the differences in growing and maturing organisms that necessitated different treatment: Ex toto non sic pueri ut viri curari debent ('In general, boys should not be treated in the same way as men'). Some of the oldest traces of pediatrics can be discovered in Ancient India where children's doctors were called kumara bhrtya. Sushruta Samhita an ayurvedic text, composed during the sixth century BC contains the text about pediatrics. Another ayurvedic text from this period is Kashyapa Samhita. A second century AD manuscript by the Greek physician and gynecologist Soranus of Ephesus dealt with neonatal pediatrics. Byzantine physicians Oribasius, Aëtius of Amida, Alexander Trallianus, and Paulus Aegineta contributed to the field. The Byzantines also built brephotrophia (crêches). Islamic writers served as a bridge for Greco-Roman and Byzantine medicine and added ideas of their own, especially Haly Abbas, Serapion, Avicenna, and Averroes. The Persian philosopher and physician al-Razi (865–925) published a monograph on pediatrics titled Diseases in Children as well as the first definite description of smallpox as a clinical entity. Also among the first books about pediatrics was Libellus de aegritudinibus et remediis infantium 1472 ('Little Book on Children Diseases and Treatment'), by the Italian pediatrician Paolo Bagellardo. In sequence came Bartholomäus Metlinger's Ein Regiment der Jungerkinder 1473, Cornelius Roelans (1450–1525) no title Buchlein, or Latin compendium, 1483, and Heinrich von Louffenburg (1391–1460) Versehung des Leibs written in 1429 (published 1491), together form the Pediatric Incunabula, four great medical treatises on children's physiology and pathology. The Swedish physician Nils Rosén von Rosenstein (1706–1773) is considered to be the founder of modern pediatrics as a medical specialty, while his work The diseases of children, and their remedies (1764) is considered to be 'the first modern textbook on the subject'. Pediatrics as a specialized field of medicine continued to develop in the mid-19th century; German physician Abraham Jacobi (1830–1919) is known as the father of American pediatrics because of his many contributions to the field. He received his medical training in Germany and later practiced in New York City. The first generally accepted pediatric hospital is the Hôpital des Enfants Malades (French: Hospital for Sick Children), which opened in Paris in June 1802 on the site of a previous orphanage. From its beginning, this famous hospital accepted patients up to the age of fifteen years, and it continues to this day as the pediatric division of the Necker-Enfants Malades Hospital, created in 1920 by merging with the physically contiguous Necker Hospital, founded in 1778. In other European countries, the Charité (a hospital founded in 1710) in Berlin established a separate Pediatric Pavilion in 1830, followed by similar institutions at Saint Petersburg in 1834, and at Vienna and Breslau (now Wrocław), both in 1837. In 1852 Britain's first pediatric hospital, the Hospital for Sick Children, Great Ormond Street was founded by Charles West. The first Children's hospital in Scotland opened in 1860 in Edinburgh. In the US, the first similar institutions were the Children's Hospital of Philadelphia, which opened in 1855, and then Boston Children's Hospital (1869). Subspecialties in pediatrics were created at the Harriet Lane Home at Johns Hopkins by Edwards A. Park. The body size differences are paralleled by maturation changes. The smaller body of an infant or neonate is substantially different physiologically from that of an adult. Congenital defects, genetic variance, and developmental issues are of greater concern to pediatricians than they often are to adult physicians. A common adage is that children are not simply 'little adults'. The clinician must take into account the immature physiology of the infant or child when considering symptoms, prescribing medications, and diagnosing illnesses. Pediatric physiology directly impacts the pharmacokinetic properties of drugs that enter the body. The absorption, distribution, metabolism, and elimination of medications differ between developing children and grown adults. Despite completed studies and reviews, continual research is needed to better understand how these factors should affect the decisions of healthcare providers when prescribing and administering medications to the pediatric population.