A fever is your brain turning up the thermostat

Deep in the brain, the hypothalamus works like a thermostat, normally holding body temperature near 37 °C (98.6 °F). A fever happens when that thermostat is deliberately turned up.

When the immune system detects an invader, it releases signalling molecules called pyrogens — including cytokines like IL-1 and IL-6, which trigger prostaglandin E2. These act on the hypothalamus and raise its set point. Convinced the body is too cold, the brain orders heat-saving and heat-making responses: blood vessels in the skin tighten and muscles shiver, pushing core temperature up to the new target.

The higher temperature is thought to help the body defend itself — stimulating white blood cells, boosting antibody production, and slowing microbes that tolerate only a narrow temperature range. In other words, the discomfort of a fever is part of the immune response working as intended.