Fever: causes, measurement, and clinical significance

Measurement and typical ranges

Body temperature can be taken at several sites and those readings differ. Oral, tympanic (ear), axillary (armpit), and rectal measurements each have characteristic offsets: rectal values are generally higher while axillary readings tend to be lower. For discussion of measurement techniques and their differences see measurement methods and site-specific comparisons. Temperatures taken under the tongue or via surface sensors may read differently; for example, a reading under the tongue is commonly lower than a rectal measurement (sublingual vs rectal).

Causes and physiology

Fever is most often a response to infection, as the immune system detects foreign organisms and releases signaling molecules that raise the hypothalamic set point. Immune pathways and inflammatory mediators are central to this response; general information about the immune system is available at immune system resources. Fever can accompany a wide range of illnesses and conditions, from routine viral and bacterial infections to noninfectious causes such as autoimmune disease or malignancy. The relationship between illness and fever is discussed further under disease context and infectious causes.

Clinical thresholds, age differences, and risks

Health professionals often categorize fever severity: low-grade fevers are roughly in the vicinity of 38 °C (100.4 °F), while higher readings may be described as moderate to high-grade. Children can develop higher fevers than adults because of differences in immune response and metabolism; pediatric guidance is available at sources such as child health. Infants and very young children require particular attention because they may deteriorate quickly. Small mammals like rabbits and cats normally have somewhat higher baseline temperatures than people; see comparative notes at rabbits and cats.

When fever is dangerous

Most fevers are self-limited, but extremely high temperatures can cause tissue injury. Temperatures approaching very high levels are clinically alarming and may require urgent care; in rare uncontrolled cases, organ damage can occur at extreme temperatures. If fever reaches severely elevated ranges, or if it is accompanied by worrisome signs, seek medical evaluation (emergency guidance). Chronic or unexplained fever may have causes such as infection or less common conditions including cancers; some malignancies that can present with fever include leukemia, lymphoma, and renal cell carcinoma.

Practical considerations and notable points

• Common causes: viral and bacterial infections are the leading triggers of fever (infectious causes).
• Measurement: be consistent in site and device to track trends rather than isolated numbers (thermometry methods).
• Management: treatments aim to address the underlying cause and relieve discomfort rather than to normalize a single temperature reading.

Historically, fever has been recognized as an important clinical sign and has guided diagnosis for centuries. Modern medicine combines observation, laboratory testing and imaging to determine cause and appropriate treatment. For accessible overviews and patient-oriented advice, see linked resources on measurement, immune response and clinical guidance (reference), (measurement), and (immunity).