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Although fever is almost always a sign of an underlying illness, many parents and patients consider it to be a serious disorder that requires immediate relief. Accordingly, fever is often a cause of great consternation for parents and is one of the most common reasons for children to visit a physician. Fever is a primitive, almost universal component of the acute phase response to illness. As such, it may result from virtually any process associated with inflammation or, more specifically, with cytokine release, including malignancies, immunologic reactions, and trauma (see Chapters 121 and 227 for a detailed discussion on thermoregulation and the genesis of the febrile response). In children, fever is most frequently a manifestation of an infectious illness (see Chapter 227), usually of benign prognosis, such as a limited viral illness or a localized bacterial infection. On occasion, however, fever may be the only clinically detectable sign of serious infection, a concern that is particularly relevant in patients whose immunologic defenses are reduced. Therefore, the appearance of fever as a lone manifestation of illness always demands a logical and careful approach to evaluating the underlying cause.

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Body temperature, even during the febrile response, follows a diurnal variation, with a nadir in the early morning and a peak in the early evening, and is influenced both by environmental circumstances and by the place where it is measured in the body. Core body temperature, the operating temperature of the tissues in vital organs (often estimated from temperature probes in the great vessels or the thoracic esophagus), is not practically accessible under most clinical conditions. Thus, families and clinicians usually rely upon measurements taken from the body surface (skin of the axilla or the forehead) or from accessible body cavities such as the mouth, the ear canal, or the rectum. In interpreting the results, it is important to remember that these measurements only reflect core temperature to the extent that the tissues from which they are obtained maintain a rich blood flow. Any decrease in perfusion of the skin or the mucous membranes lining the cavities where the measurement is obtained will result in an underestimation of core temperature. Many pediatricians define fever as a rectal temperature measurement of 38°C or greater, obtained with a reliable measurement tool, such as an electronic or glass and mercury thermometer.1

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Illnesses associated with fever are particularly common in the first few years of life. Children younger than 3 years of age and especially young infants are at greater risk of suffering from a serious infection because of their limited immunologic repertoire and the frequent lack of localizing signs (see Chapter 227 for a discussion on common pathogens).

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The management of the child with acute fever is discussed in Chapter 227. Management of the child with chronic fever of unknown origin is discussed in Chapter 228, and of fever in the immunocompromised child in Chapter 229. An algorithm ...

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