Understanding a Fever
By Lauren Amphlett

A fever, also known as pyrexia, is a common symptom that can occur in various illnesses and conditions. But what exactly is a fever, and why do we get one? It’s important to understand this symptom, as it’s often a sign that our bodies are fighting an infection.

What is a Fever?

A fever is an increase in your body’s normal temperature range, usually in response to an illness or infection. While the average body temperature is typically around 36.5 to 37.2 degrees Celsius, a fever is generally agreed upon in the medical field to be a body temperature of 38 degrees Celsius or above.

Why Do We Get a Fever?

Fever is not an illness in itself but rather a symptom, often indicative of an underlying condition. It’s essentially a defence mechanism of our bodies. When bacteria or viruses invade our systems, our immune system responds by increasing our body temperature. This elevated temperature helps the body to kill off the invading organisms, as many of them cannot survive in higher temperatures.

Moreover, a fever also boosts the body’s immune response by stimulating the production of white blood cells, the soldiers of our immune system, and other substances that help fight off infections.

What Constitutes a Fever?

While it’s important to remember that everyone’s ‘normal’ body temperature can vary slightly, the NHS categorises a fever in adults as a body temperature of 38 degrees Celsius or higher. However, even a slight elevation in temperature may cause discomfort and is usually the first sign of an illness. Please note it’s not always easy to take a temperature accurately.  If you feel hot or shivery, you may have a high temperature even if a thermometer says your temperature is below 38C.

Normal Temperature Limits

While the average body temperature for a healthy adult typically ranges between 36.5 and 37.2 degrees Celsius, it’s normal for your body temperature to fluctuate throughout the day. For instance, it tends to be lower in the early morning and higher in the late afternoon and evening.

However, if your body temperature exceeds 38 degrees Celsius, it’s usually a sign that your body is fighting an infection or illness. In the case of a very high fever of 39.5 degrees Celsius or higher, it’s crucial to seek medical attention immediately, as this could indicate a severe infection.

Why Do We Shake During a Fever?

Fever often comes with chills or shivering, which can seem paradoxical when your body is overheated. This shaking is essentially a side effect of your body trying to increase its temperature. The process is triggered by the hypothalamus, our body’s thermostat, which instructs our muscles to contract and relax rapidly to generate warmth, leading to what we recognise as shivering.


So, while fevers can be uncomfortable, they are often a sign that your body is doing exactly what it needs to do to fight off an infection or illness. However, it’s important to monitor a fever closely and seek medical help if the fever is high, persistent, or accompanied by other severe symptoms such as confusion, difficulty breathing, or severe pain.

You can read more on the NHS.uk website here.