What Is a Fever? The Body Moving Its Own Thermostat

Temperature is regulated by the hypothalamus, which behaves like a thermostat comparing the body's actual temperature against a target and triggering heating or cooling responses to close the gap. During infection, immune cells release signalling molecules called pyrogens, and these act on the hypothalamus through prostaglandins to raise the target. Nothing is broken; the control system is working exactly as designed towards a higher number. That explains the sequence people experience. While the actual temperature is below the new target, the body reads itself as too cold and responds accordingly, so you shiver, your blood vessels constrict, your skin goes pale and you want blankets, which is the chill phase. Once the new temperature is reached you feel neither hot nor cold. When the infection resolves and the target drops back, you are suddenly too hot by your own standard, so you sweat and flush, which is the breaking of the fever. Fever-reducing drugs work by blocking the prostaglandin production that raised the target, which is why they lower the setpoint rather than cooling you directly.

Raising body temperature is metabolically expensive and it is conserved across a wide range of species, including reptiles that achieve it behaviourally by seeking warmer places, which is strong evidence that it does something useful:

• •Many bacteria and viruses replicate more slowly at higher temperatures, narrowing the window in which they can multiply
• •Several immune functions work better warm, including the movement of white cells to sites of infection and the proliferation of lymphocytes
• •Iron and zinc availability in the blood falls during the response, restricting nutrients that bacteria need
• •Heat shock responses in infected cells can make them more visible to immune surveillance
• •Experimental work in animals shows that blocking fever during some infections worsens outcomes, and observational studies in humans have suggested similar patterns in certain serious infections
• •The evidence is strong enough that fever is generally treated as a defence rather than a symptom to be abolished, while remaining weak enough that routine treatment for comfort is not considered harmful

The familiar figure of thirty-seven degrees Celsius as normal comes from nineteenth-century measurements and is a rounded average that conceals a great deal of variation. Normal temperature varies between individuals, varies by around half a degree through the day with a low in the early morning and a peak in the late afternoon, varies with the menstrual cycle, and varies by measurement site, with rectal readings higher than oral and oral higher than armpit. Large modern datasets also suggest average body temperature has fallen slightly over the past century, possibly reflecting lower rates of chronic inflammation and infection in the population. Thresholds for calling something a fever are therefore conventions rather than biological boundaries, usually set around thirty-eight degrees. The height of a fever is a poor guide to how serious an illness is, since ordinary viral infections routinely produce high readings and some very serious infections produce little or none, particularly in older adults, in infants and in people whose immune response is suppressed, where an absent fever can itself be a warning sign.

Most fevers are self-limiting responses to ordinary infections and need no more than fluids and comfort. The situations that genuinely warrant urgency are specific rather than general: any fever in a very young infant, a fever with a rash that does not fade under pressure, a fever with severe headache and neck stiffness or with confusion, a fever in someone whose immune system is compromised or who has recently travelled to a malaria region, a fever accompanied by breathing difficulty, and a fever lasting longer than expected without explanation. Very high temperatures from heatstroke are a completely different mechanism, since the setpoint is normal and the body has simply been overwhelmed by external heat and cannot shed it, which is a medical emergency requiring physical cooling, and fever-reducing drugs do not help because there is no raised setpoint to lower. Febrile convulsions in young children are frightening to witness, are usually brief and harmless, and are related to the rate at which temperature rises rather than to how high it gets, which is why preventive dosing does not reliably prevent them.

Immune signalling molecules act on the hypothalamus to raise the temperature the body aims for, so you shiver while climbing to it and sweat when the target drops back. The response slows many pathogens and improves several immune functions, which is why it is conserved across species. The thirty-seven degree figure is a rounded average that varies by person, time of day and measurement site, and how high a fever goes says little about how serious the illness is.