Why do children's temperatures go so high?

Because children's thermoregulation is immature and their immune responses are vigorous — and, crucially, the height of a fever is a poor guide to how serious the illness is, which is the opposite of what most parents assume.

Why children run higher fevers than adults:

Higher surface area relative to body mass, so heat exchange with the environment is proportionally larger in both directions.

Less developed temperature regulation, particularly in infants.

A busier immune system. Young children meet an enormous number of pathogens for the first time, and a first encounter produces a stronger response than a repeat one.

Higher metabolic rate.

What a fever is doing. It is a deliberate resetting of the body's thermostat by the hypothalamus in response to immune signalling. Many pathogens replicate less well at higher temperatures, and immune function improves. It is a defence, not the disease.

The point that matters most: the number is not the danger. A very high temperature can accompany a minor viral illness, and a seriously unwell child — particularly a young infant — can have a normal or low temperature. Clinicians assess the child, not the thermometer.

What actually indicates concern, following NHS guidance: difficulty breathing or rapid breathing; a rash that does not fade when pressed with a glass; unusual drowsiness, or being difficult to wake; a weak, high-pitched or continuous cry; mottled, blue or very pale skin; a stiff neck or dislike of light; a fit; signs of dehydration such as far fewer wet nappies; refusing feeds; or simply a child who seems much more unwell than usual.

Age changes the threshold considerably. For babies under three months, a temperature of 38°C or above warrants urgent medical assessment regardless of how well they seem, and guidance is stricter again for newborns.

On treatment: paracetamol and ibuprofen are given to make a child more comfortable, not to bring the number down for its own sake, and there is no need to treat a fever in a child who is otherwise content. Follow the dosing for their age and weight.

Febrile convulsions are frightening but usually brief and not caused by the fever's height.

General information — contact 111, your GP or 999 if you are worried.