A fever is any significant increase in an animal's body temperature triggered by its immune system.
In the past, a standard of 37 degrees Celsius (89.6 degrees Fahrenheit) has stood as a landmark for healthy 'normal' human body temperatures. This has been challenged in recent years, with anywhere from around 36 to just over 37 degrees Celsius (97 to 99 Fahrenheit) expected in a typical adult, and children running a touch warmer with common temperatures of up to 38 degrees Celsius (about 100 Fahrenheit).
Scientifically speaking, there is no specific degree of change in temperature universally considered as a fever. Limits vary between different medical systems around the globe and are often based on age and history.
In typically healthy adults, a rise of a degree or so isn't considered a major cause for medical attention.
Results exceeding around 39 degrees Celsius (102 Fahrenheit), however, could prompt a need to investigate potential causes and treatments. Temperatures higher than 40 degrees Celsius (104 Fahrenheit) are often considered life-threatening and in need of reducing.
Among the elderly or people undergoing chemotherapy treatments, a temperature beyond 37.5 degrees Celsius or so (99.5 Fahrenheit) could be viewed as an indication of infection requiring immediate action.
Children are more susceptible to fevers while their immune system continues to develop. To be on the safe side, parents are commonly encouraged to seek medical assistance if temperatures exceed 38 degrees Celsius (100.4 degrees Fahrenheit).
How do fevers develop?
The body's preferred temperature range is managed by a part of the nervous system called the hypothalamus.
This small nub of brain tissue generates various biological responses that increase or decrease temperatures, such as by burning glucose through increased muscle movements (shivering) or increasing skin evaporation (sweating).
As the body's thermostat, the hypothalamus can also have its preferred temperature changed by materials known as pyrogens.
These are molecules typically produced by bacteria, viruses or even white blood cells, which cause the hypothalamus to reset the body's ideal temperature at a higher limit.
Why do we get fevers?
For hundreds of millions of years, animals have managed infections by behaviourally or metabolically raising their body temperature.
Exactly how this increased warmth fights off invaders isn't clear. There is little evidence to suggest that a such a moderate jump in temperature is of direct threat to disease-causing viruses and bacteria.
One way a fever seems to benefit us is by enhancing the activity of certain immune responses, favouring movement and delivery of key chemicals and immune cells to target sites around the body.
In other words, some parts of our immune system have simply evolved to work best when the heat is on.
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