| Fever is an abnormal elevation of body temperature that occurs as part of a specific biologic response that is mediated and controlled by the central nervous system. (See 'Pathogenesis' below.) The temperature elevation that is considered "abnormal" depends upon the age of the child and the site of measurement. The temperature elevation that may prompt clinical investigation for infection depends upon the age of the child and the clinical circumstances (eg, immune deficiency, sickle cell disease, ill-appearance, etc); in most scenarios, the height of the fever is less important than other signs of serious illness (eg, irritability, meningismus) [35-38]. ●In the otherwise healthy neonate (0 to 28 to 30 days of age) and young infant (one to three months of age), fever of concern generally is defined by rectal temperature ≥38.0°C (100.4°F). (See "Febrile infant (younger than 90 days of age): Definition of fever", section on 'Definition of fever'.) ●In children 3 to 36 months, fever generally is defined by rectal temperatures ranging from ≥38.0 to 39.0°C (100.4 to 102.2°F) and fever of concern by rectal temperatures ≥39.0°C (102.2°F) if there is no focus of infection on examination. (See "Fever without a source in children 3 to 36 months of age", section on 'Fever of concern'.) ●In older children and adults, fever may be defined by oral temperatures ranging from ≥37.8 to 39.4°C (100.0 to 103.0°F) and fever of concern by oral temperatures ≥39.5°C (103.1°F). Oral thermometers average one half of a degree Celsius lower than rectal. The above is pretty much straight out of UTD, as I am far too lazy to retype it for anyone’s benefit. |