Why does your body raise its temperature when you're sick?

A fever is your body deliberately turning up its own thermostat. The brain's hypothalamus normally holds body temperature at a steady set point, but when it detects a substance called a pyrogen, often released as part of the immune response to bacteria or viruses, it raises that set point higher. The body then works to catch up: blood vessels near the skin constrict to hold heat in (which is why you feel cold even as your temperature climbs), muscles shiver to generate warmth, and brown fat tissue burns energy specifically to produce heat.

This isn't a malfunction: the heat is thought to help defend you. Many pathogens have strict temperature requirements to reproduce, so a higher temperature can hinder them, while some important immune reactions run faster when it's warmer. Studies in warm-blooded animals suggest they recover more quickly from infections because of fever, and in sepsis, fever is associated with lower mortality. A fever in response to an infection is generally regarded as protective, whereas fever without infection may be maladaptive. It is also one of the most common medical signs, part of about 30 percent of children's healthcare visits, and it often worries parents and health professionals more than it usually deserves, a phenomenon known as 'fever phobia'.

The strategy isn't limited to animals with an immune-system-based fever. Some grasshopper species thermoregulate to reach body temperatures 2 to 5°C above normal, which inhibits the growth of fungal pathogens. Honeybee colonies, too, can induce a fever in response to a fungal parasite, Ascosphaera apis.

Fever does have limits. Hyperpyrexia, a core temperature of 40 or 41°C or more depending on the source, is considered a medical emergency, because it may signal a serious underlying condition or lead to severe harm, including permanent brain damage, or death.