Pyrexia
The medical term for fever — a body temperature raised above the normal range.
What it means
Pyrexia is a raised body temperature, conventionally taken as 38°C or above. From the Greek pyr, fire.
It is produced by the body deliberately resetting its own thermostat in response to infection or inflammation — which is why the shivering phase of a fever feels cold despite the temperature being high.
Fever is a sign, not a diagnosis. The height of a temperature correlates poorly with how serious the underlying cause is, particularly in adults.
In practice
"Apyrexial" means without fever. "Pyrexia of unknown origin" describes a prolonged fever with no cause identified after initial investigation.
Notes may record a patient as "febrile" or "afebrile".
Why it matters
Treating a fever with paracetamol makes a person feel better but does not treat the cause and does not shorten the illness. Fever should not be treated purely to bring a number down.
Fever in a baby under three months, fever with a rash that does not fade under pressure, or fever with confusion, severe breathlessness or difficulty rousing, needs emergency assessment.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026