What happens in your body during a fever?

In response to infection, the hypothalamus in the brain can induce a temporary spike in body temperature — a fever — in a variety of ways.  (Image credit: Highwaystarz-Photography via Getty Images)

You wake up in the middle of the night, shivering. You're experiencing a fever — a temporary spike in body temperature. 

Fevers can arise as the body's defense system fights off an infection, but they can also be triggered by other things, including autoimmune diseases such as rheumatoid arthritis, or occur as a side effect of certain drugs.

But what happens in the body during a fever?

Human body temperature varies slightly from day to day and from person to person, but it's normally maintained at around 98.6 degrees Fahrenheit (37 degrees Celsius). This creates the perfect environment for our cells to work efficiently. Part of the brain called the hypothalamus acts like a thermostat, constantly monitoring the body's temperature and turning internal dials to rein it back to roughly 98.6 F. 

Related: Why do coughs linger after a cold?

During an infection, when our immune cells detect foreign invaders such as bacteria or viruses, they release fever-inducing chemicals called pyrogens. These chemicals travel to the brain, where they act on temperature-sensitive neurons in the hypothalamus, essentially telling it that it's time to raise the temperature in the house, Dr. Paul O'Rourke, an assistant professor of medicine at Johns Hopkins University, told Live Science. 

As a result, these neurons release hormone-like substances called prostaglandins — specifically, one called PGE2 — to twist the dial on the body's thermostat and initiate a fever. 

"We typically consider a fever when you're reaching temperatures greater than 38 degrees Celsius [100.4 F]," O'Rourke said. 

The hypothalamus can raise body temperature in several ways. For instance, it directs blood vessels to constrict, which reduces the amount of heat that dissipates through the surface of the skin. It also induces shivering to generate as much heat as possible. 

These physiological processes collectively form part of the body's first line of defense against infection, known as acute inflammation. The main aim being to bring infection under control and stop it from spreading. 

Paradoxically, people may have chills alongside a fever, even though their body temperature is rising. This is because the hypothalamus has temporarily increased the body's internal thermostat to a higher "normal" level. As your body tries to reach this new baseline, you feel comparatively cold. 

So why does the body need the heat?

One possible reason is to make it harder for bacteria or viruses to replicate and infect our cells, O'Rourke said. A higher body temperature may also turn the immune system into a better "fighting machine," he said. For example, when our body temperature rises, cells produce heat shock proteins (HSPs), which activate immune pathways to fight off infection. HSPs are normally upregulated by cells during inflammation, as the body strives to protect itself against foreign invaders. 

"For your average older child or adult, you may experience a degree of fever for a few days, certainly two or three days, without necessarily needing to get a lot of medical attention," Dr. Kitty O'Hare, a consulting associate in the Department of Medicine at Duke University, told Live Science. 

But if you are concerned about your symptoms or they don't seem to be improving, you should contact your healthcare provider, she said. 

Sometimes when children get a high fever, for instance, they can experience convulsions called febrile seizures. Although these may be frightening, they generally last only a few minutes and are usually harmless. Nevertheless, parents should call their healthcare provider anytime their child has a seizure, even if it is during a fever, O'Hare said. 

The degree of fever also matters, O'Hare said. "It's good to get advice from your own health care provider based on your health history on how much of a fever would be problematic for you," she said. 

Depending on your age, an over-the-counter medication such as acetaminophen or ibuprofen can help alleviate the symptoms of fever. Removing a layer of clothing, taking a cold bath and drinking cool liquids can also help the fever to improve, she said. 

Increasing your body temperature during a fever takes a lot of effort — for every 1.8 F [1 C] increase in body temperature, you expend an additional 10% more energy than you would normally use to maintain your temperature. So it is important to stay well rested, O'Rourke said. 

This article is for informational purposes only and is not meant to offer medical advice.

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Emily Cooke
Staff Writer

Emily is a health news writer based in London, United Kingdom. She holds a bachelor's degree in biology from Durham University and a master's degree in clinical and therapeutic neuroscience from Oxford University. She has worked in science communication, medical writing and as a local news reporter while undertaking journalism training. In 2018, she was named one of MHP Communications' 30 journalists to watch under 30. (emily.cooke@futurenet.com