A fever is when the body temperature goes up because of its immune system.
In the past, people thought 37 degrees Celsius (89.6 degrees Fahrenheit) was a normal temperature for humans. But now, we know normal can be from about 36 to just over 37 degrees Celsius (97 to 99 Fahrenheit) for adults. Children often have slightly higher temperatures, up to 38 degrees Celsius (about 100 Fahrenheit).
There's no exact temperature that's the same for everyone to say it's a fever. It can be different depending on where you are in the world and things like your age and health history.
For healthy adults, a small increase in temperature is usually not a big worry.
But if your temperature goes above 39 degrees Celsius (102 Fahrenheit), it might be time to find out why and how to treat it. Over 40 degrees Celsius (104 Fahrenheit) is very serious and needs to be lowered quickly.
Older people and those having chemotherapy should be careful if their temperature goes over 37.5 degrees Celsius (99.5 Fahrenheit), as it might mean an infection.
Children can get fevers more easily while their immune system is still growing. Parents should get medical help if a child's temperature is over 38 degrees Celsius (100.4 Fahrenheit).
How do fevers start? The hypothalamus in our brain controls our body temperature.
This small part of the brain can make us warmer or cooler, like by making us shiver or sweat.
The hypothalamus can change what temperature it thinks is best if it detects molecules called pyrogens. These can come from bacteria, viruses, or even our own white blood cells.
Why do we get fevers? Animals have been raising their body temperature to fight infections for millions of years.
It's not totally clear how this helps, but it doesn't seem to directly harm viruses and bacteria.
A fever can help by making some parts of our immune system work better, helping to move important chemicals and cells to where they're needed in the body.
In short, some parts of our immune system do better when our body is warmer.
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