Why does your body get a fever?
A fever is your body turning up its own thermostat on purpose. When your immune system detects an infection, your brain raises your target temperature, which makes it harder for germs to thrive and helps your defenses work faster.
Your immune system spots invaders
When viruses or bacteria get into your body, immune cells detect them and release chemical messengers into your blood. Some of those messengers travel straight to your brain.
Share this stepYour brain turns up the heat
Deep in your brain, an area called the hypothalamus works like a thermostat. The messengers make it raise your target temperature, often from about 37 °C (98.6 °F) to 38–39 °C (100–102 °F).
Share this stepWhy you feel cold while burning up
Your body is suddenly colder than its new target, so you feel chilly. Blood vessels in your skin narrow to keep heat in, and your muscles shiver to make more, until you reach the new temperature.
Share this stepA fever is a defense, not a malfunction.
Infectionhigher thermostatstronger defense
Many germs grow more slowly when it’s warmer, and immune cells work faster. When the infection fades, the thermostat resets, and you sweat to cool back down. That’s the fever “breaking”.
Common follow-up questions
Should I always bring a fever down?
Not necessarily. A mild fever is part of fighting infection. Fever medicine mainly helps you feel more comfortable. Follow the dosing on the package.
When should I contact a doctor?
If the fever is very high, lasts more than a few days, or comes with a stiff neck, trouble breathing, confusion or a rash. Babies under 3 months with any fever should be checked right away.
Why do I sweat when the fever breaks?
Your thermostat drops back to normal, so your body is now too warm for its target. Sweating cools you down.
Is a fever the same as being hot from exercise?
No. In a fever your brain raises its target on purpose. When you overheat from exercise or sun, the target stays normal and your body is struggling to cool down.
