Fever

Fever is a temporary elevation of body temperature above the normal range, typically defined as a measured core temperature of 38°C (100.4°F) or higher. It occurs when the hypothalamic set point is raised in response to pyrogens—either exogenous substances such as microbial components, or endogenous cytokines such as interleukin-1, interleukin-6, and tumor necrosis factor-alpha released by immune cells. The higher set point triggers heat production and conservation mechanisms, including shivering and reduced heat loss; as the new set point is reached, patients may feel cold and may shiver. Once the illness resolves and the set point returns to normal, sweating and vasodilation occur (defervescence).

Fever is a symptom, not a disease, and can accompany infections (viral or bacterial), inflammatory or autoimmune disorders, malignancy, medications, or heat exposure. In many cases the exact cause remains unknown. Fever of unknown origin is defined as a fever higher than 38.3°C (101°F) on multiple occasions lasting at least three weeks with no diagnosis after initial evaluation.

Evaluation typically includes history, physical examination, and directed laboratory testing (eg, complete blood count, inflammatory markers such as C-reactive protein or ESR, cultures or imaging as indicated by age and risk). Management focuses on treating the underlying cause and relieving symptoms. Antipyretics such as acetaminophen or ibuprofen are commonly used to reduce fever and discomfort; aspirin is avoided in children. Adequate hydration and rest are advised, and antipyretics should not be used to mask serious illness when red flags are present.

Most fevers in healthy individuals resolve with care, but fever can be more serious in young children, older adults, or immunocompromised patients. Febrile seizures are possible in children with high fever.