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What fever is

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This page has information on fever, what it is and what happens to your body when you have a fever. You can go straight to sections on



Having a fever means that your body temperature is higher than normal. Something inside your body, such as an infection, has caused your temperature to go up.

A part of the brain called the hypothalamus controls your body temperature.

Diagram showing the parts of the brain

Normally, the hypothalamus keeps your temperature at around 37ºC (98.6ºF). This can vary depending on the time of day. Your temperature is usually lowest in the early hours of the morning and highest in mid afternoon. But generally, it stays around 36.5 and 37ºC.

When you have a fever, your body temperature rises above 38ºC (100.4ºF). This usually means there is something wrong somewhere. Fever is a common symptom in people with all types of cancers. It can be very uncomfortable and cause a lot of concern for you and those looking after you.

If you have cancer and develop symptoms of a fever or infection, it is very important to let your doctor know. It may be nothing, but it could be a sign of a very serious infection. The earlier an infection or fever is treated, the less likely it is that you will have more serious complications. It is very important to find out what is causing the fever so that it can be treated quickly and in the best possible way.


Low grade fever

If you have a rise in temperature that is between 37ºC and 38ºC, it is often called a low grade fever. It can also be a sign that something is wrong, particularly if raised for some time. A low grade fever may not always need treatment. But this can depend on what treatment for cancer you are on. If you are having treatment that affects your immune system, it might be important to let your doctor know about your temperature. It may not be anything to worry about, but your doctor may want to keep a closer eye on you.


The 3 phases of fever

Even though having a fever is uncomfortable, it is not a bad thing. It is your body’s way of letting you know something is wrong. In a way, the fever is helping to fight off your infection. This happens in 3 phases

Your blood and lymphatic system make white blood cells, which fight infection. When you have an infection, you make lots and lots of these cells. They work faster and faster to try and fight off the infection. The increase in these white blood cells affects the part of your brain that controls your body temperature (the hypothalamus). This makes your body heat up, causing a fever. In the early stages of a fever, you often feel cold and start to shiver. This is your body’s response to a rising temperature – the blood vessels in your skin tighten up (constrict), forcing blood from the outer layer of your skin to inside your body where it is easier to keep the heat in. The outer skin layer then becomes cool and your muscles start to contract. This makes you shiver. Shivering produces more heat and raises your temperature even more.

In the second phase of a fever, the amount of heat you make and lose is the same. So the shivering stops and your body remains at its new high temperature.

Your body starts to try and cool down so that your temperature can return to normal. The blood vessels in the skin open again, so blood moves back to these areas. You may sweat, as this helps to cool down the body.

This phase of a fever may or may not happen naturally. You may need to have some medication to start it off, as well as treating the underlying cause of the fever.


Who is most at risk of having complications from a fever

The very young and elderly are more likely to get complications from a fever. In the elderly, the part of the brain that regulates temperature (the hypothalamus) does not work as well as it does in the young. The body temperature can rise too much, causing heart problems and confusion.

Children under six may have a fit (seizure) if their temperature gets too high. But in most people, the cause of the fever – such as infection – is more likely to cause problems than the fever itself.

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Updated: 23 April 2014