What does a fever actually do?

A fever is not the illness. It is a deliberate, regulated response — your body raising its own thermostat setting because a higher temperature makes it a worse environment for pathogens and a better one for your immune system.

The control point is the hypothalamus. Signalling molecules released during infection instruct it to raise the target temperature. Everything else follows from that new set point: shivering generates heat, blood vessels in the skin constrict to retain it, and you feel cold because your body now considers your normal temperature too low. The chills at the start of a fever are the mechanism working, not a separate symptom.

What the raised temperature achieves: many bacteria and viruses replicate less efficiently above normal body temperature, immune cell mobility and activity increase, and some pathogen defences work less well. Fever is evolutionarily ancient and appears across vertebrates, which is a strong signal that it is useful.

Should you always bring it down? Not necessarily for its own sake. Current guidance in several countries is to treat for comfort rather than to chase a number — antipyretics such as paracetamol or ibuprofen are appropriate if the person feels miserable, but a fever alone in someone who is otherwise coping does not require suppression. Reducing a fever does not shorten the illness.

Height is a poor guide to severity, particularly in children. How the person looks and behaves matters far more than the reading.

Seek medical help urgently for: any fever in a baby under three months; a stiff neck, severe headache, a rash that does not fade under pressure, breathing difficulty, confusion, seizure, or a child who is floppy, unusually drowsy or hard to rouse; fever lasting more than a few days; or fever in someone immunosuppressed.

General information only — follow local medical guidance.