Fevers are uncomfortable, annoying and occasionally dangerous. But they are also an essential part of how we protect ourselves.

It's 3am and you can't sleep. Sweats and shivers let you know something is not quite right. A furnace-like heat races to your forehead, shivers and chills trickle down your spine. You feel helpless, confused and exhausted. "It's only a fever," you tell yourself.

An evolutionary feature more than 600 million years old, fevers are a common accompanyment of a wide range of infections by viruses, bacteria and fungi. Many of us will have experienced them during a bout of flu, for example. Fever has also been a sign of such serious, and often deadly, illnesses through human history that we have incorporated it into the name we give many diseases – scarlet fever, dengue fever, yellow fever, Lassa fever. The list goes on. (Learn more about how viruses get their names in this article by Sarah Pitt.)

Despite this, humans only reached a full understanding of how our bodies produce fever in the 20th Century.

So why exactly do we get them, should we always treat them – and at what point do they go from beneficial discomfort to a serious problem?

Our ancestors were fully aware of the danger of fevers, and they fed into interesting ideas of how the body works, says Sally Frampton, humanities and healthcare fellow and historian of medicine at the University of Oxford.

"Today we would know, 'Oh you've got a fever, there's something else going on'," she says. "But for a lot of [people] in the early modern world, and up to the 19th Century, there was a sense that the fever is the disease."

The ancient Greeks treated fevers with everything from starvation to bloodletting, both of which were used up to the 19th Century to try to cool fevers. The big shift in our understanding of fever came after germ theory arrived, says Frampton, when we learned more about infections and fever started being seen as a symptom rather than a disease itself.

Germ theory

Germ theory, first published by Louis Pasteur in 1861, identified microorganisms invading our bodies as the cause of disease. The French scientist paved the way towards our understanding of microbial infections as something we can prevent through cleanliness.

After a surge in maternity deaths during childbirth due to "childbed fever" (now known as postpartum infection) in a Parisian hospital back in 1875, Pasteur proposed that the infection was being spread by physicians and hospital attendants. He promptly told clinicians to wash their hands and sterilise their tools with heat.

We now know fever is part of our bodies' innate response to infection. Found across the animal world in both warm-blooded and cold-blooded vertebrates, the chills when fever settles in, followed by the sticky and relentless sweats when it breaks, is our body's intruder alarm – and attack – system.

Fevers signal that pathogens and other hostile actors are making our bodies their home – and that we're putting up a fight. Unpleasant as they may be, they help us to get rid of these intruders. When unchecked, though, fevers can become harmful.

A fever is generally characterised by a body temperature above 38C (100F). It can occur as our body's response to infections, but it can also be triggered by auto-immune diseases, inflammatory diseases or after a vaccination.

As our bodies react to the threat of a virus or pathogenic microorganism, such as fungal or bacterial infections, our core........

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