Science Puzzle
Why Do You Get a Fever?
When you have an infection, your temperature often rises from about 37 °C to 38 or 39 °C. Strangely, while it's going up you can feel freezing cold and shiver, even under a thick duvet.
Is a fever something the germs do to you, or something your body does on purpose? And why?
The Answer
Your body does it on purpose. When your immune system detects an infection, it signals your brain to turn up your body's thermostat. A slightly hotter body helps your immune system and makes life harder for many germs.
A small region at the base of the brain, the hypothalamus, acts like a thermostat, normally keeping you at about 37 °C. Immune cells fighting an infection release chemical messengers that travel to it and raise its setting. Suddenly 37 °C counts as too cold, so the body does what it always does when it's too cold: you shiver, which makes your muscles produce heat, and the blood vessels in your skin narrow, so your hands and feet feel cold. You feel chilly until your temperature reaches the new setting.
When the infection is beaten, the setting drops back to normal. Now you're too hot for it, so you sweat and throw off the covers: the fever "breaks".
Many bacteria and viruses multiply more slowly a couple of degrees above normal, while parts of the immune system work faster. Even cold-blooded animals do it: lizards with an infection deliberately move to warmer spots to raise their temperature. A very high fever can be dangerous, though, which is why doctors keep an eye on it.
The principle: A raised set point. In response to infection, the brain turns up the body's thermostat, so the body heats itself to a temperature that helps the immune system and hinders many germs.