Why do we get a fever?

Fever is a deliberate defence, not a side effect of illness. When the immune system detects a pathogen it releases chemical signals that raise the set point of the thermostat in the brain’s hypothalamus — the body then treats a higher temperature as its new normal.

The raised temperature helps in two ways: many bacteria and viruses replicate more slowly, and immune cells work more actively. That is why suppressing every fever is not automatically the best strategy.

The chills and shivering at the start of a fever follow from the same logic. The set point has risen but the actual body temperature has not caught up, so the body reads itself as too cold and shivers to generate heat.

None of which makes fever something to shrug off. A very high or prolonged fever, a fever in a small child, breathing difficulty, seizures or severe weakness all need medical attention — the cause has to be identified, not just the temperature brought down.