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

What fever is

When you have a fever, your body temperature rises above 37.5C (99.5F). This usually means there is something wrong somewhere.

A part of the brain called the hypothalamus controls your body temperature. Normally, the hypothalamus keeps the temperature of the inside of your body at around 37C (98.6F). This can vary depending on the time of day and what you're doing. But generally, it stays around 36.5 and 37C.

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 might be nothing, but it could be a sign of a very serious infection.

What to do

If you're having cancer treatment and your temperature goes above 37.5C call your advice line straight away. You should have a number to call them on 24 hours a day. 

The earlier an infection or fever is treated, the less likely it's that you will have more serious complications. It's very important to find out what is causing the fever so that it can be treated quickly and in the best possible way.

The 3 phases of fever

Fever 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 body reacts and heats up

Your blood and lymphatic system make white blood cells, which fight infection. When you have an infection, you make lots of these cells. They work faster to try and fight off the infection.

The increase in these white blood cells affects your 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.

The fever levels off

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.

Cooling down

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 sweat which helps to cool the skin, 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

The very young and elderly are more likely to get complications from a fever. In the elderly 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 6 might 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.

Last reviewed: 
05 Aug 2019
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  • The information on this page is based on literature searches and specialist checking. We used many references and there are too many to list here. If you need additional references for this information please contact patientinformation@cancer.org.uk with details of the particular risk or cause you are interested in. 

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