Fever

For a lay person:

A fever is when your body temperature rises higher than normal — usually above about 100.4°F (38°C) — as part of your immune system’s defense against infection or illness. It’s like your body turning up the thermostat to make it harder for germs to survive and to help your immune cells work better. You might feel hot, sweaty, chilled, tired, or achy, and your skin can feel warm or flushed. Fevers are very common with colds, flu, ear infections, or other bugs, and they often go away on their own as your body fights off the cause. While uncomfortable, a fever itself is usually helpful — not harmful — unless it gets very high or lasts too long, in which case you should see a doctor.

For a trained medical person:

Fever (pyrexia) is a regulated elevation of core body temperature above the normal daily range, resulting from a upward shift in the hypothalamic thermoregulatory set point triggered by pyrogenic cytokines (e.g., IL-1, IL-6, TNF-α) in response to infection, inflammation, or other stimuli. It is distinguished from hyperthermia, which is an unregulated rise in temperature due to excessive heat production or impaired heat loss.

Standard Thresholds (oral measurement, most common reference):

• Normal core body temperature: approximately 36.7°C (98.0°F) average, with a physiologic range of ~35.3–37.7°C (95.5–99.9°F), varying by time of day (lower in AM, higher in PM), age, and site.
• Fever: generally defined as ≥38.0°C (100.4°F) core temperature. Some sources use ≥37.8°C (100.0°F) orally.

Temperature Measurement Sites (approximate equivalents):

• Rectal: highest (gold standard in infants); +0.5–1°C above oral.
• Oral: standard for adults.
• Tympanic, temporal, axillary: lower reliability; adjust interpretation accordingly.

Grading of Fever Severity:

• Low-grade: 37.3–38.0°C (99.1–100.4°F)
• Moderate-grade: 38.1–39.0°C (100.6–102.2°F)
• High-grade: 39.1–41.0°C (102.4–105.8°F)
• Hyperpyrexia: >41.0°C (>105.8°F) — medical emergency.

Etiologies: Infectious (most common), autoimmune, malignancy (e.g., tumor fever), drug-induced (drug fever), tissue injury, or factitious. Evaluation includes history, physical exam, labs (CBC, CRP/ESR, cultures, imaging as indicated), and consideration of fever of unknown origin (FUO) if prolonged (>3 weeks) without diagnosis after initial workup.

Physiologic Benefits & Risks: Enhances immune function (e.g., increased leukocyte activity, interferon production) but can increase metabolic demand, cause discomfort, dehydration, or (in children) febrile seizures. Antipyretics (acetaminophen, ibuprofen) are used for comfort, not routinely for temperature reduction alone unless >39–40°C or in specific high-risk patients. Persistent or high fever requires investigation of underlying cause.