Fever

What can cause a fever?

There are a few possible causes of fever, the most common is an infection. Some cancer treatments can make you more at risk of infection. Other causes of a fever include the cancer itself or having a reaction to a drug. Sometimes the cause of the fever might be unknown.

What is a fever?

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 between 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 not be serious. But in some situations, it could be a sign of a very serious infection.

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

What is an infection?

An infection is an invasion and growth of germs in the body. The germs could be:

• bacteria

• viruses

• yeast

• fungi or other microorganisms

Infections can start anywhere in the body and can spread throughout the body. Depending on where the infection is you may get a fever. 

Read about symptoms of infections and where infections commonly start

How the body reacts to an infection

Fever is your body’s way of letting you know something is wrong. The fever helps 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. They are part of our immune system which fights infection. When you have an infection, you make lots of these cells. They work faster to try and fight off the infection. 

How we develop a fever is very complex. When our immune system recognises harmful bacteria or viruses the hypothalamus releases a hormone in response to this. The blood vessels in our skin start to narrow (constrict) so we lose less body heat. And the muscles contract, this makes you shiver. Both these cause the body to warm up and our temperature starts to rise. 

An increase in body temperature makes it harder for the bacteria and viruses that cause infections to survive. So, it helps your body fight the infection. 

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

In the third phase, 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 and cool down your body.

This phase of a fever may or may not happen naturally. You may need to have some medication to help relieve the symptoms, as well as treating the 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.

Read about the causes of fever