Fever Dr Trisha Macnair A fever, or pyrexia, is when the body's temperature rises above normal. In many cases it resolves as the cause disappears, but in some instances it can be a sign of something more serious.

In this article

What's a normal temperature? What causes it?

What's the treatment? When to contact a doctor

Normal body temperatures can vary and are influenced by factors such as exercise, eating, sleeping and the time of the day - the lowest temperature is usually recorded at around 3am, the highest at 6pm.

The average normal body temperature taken in the mouth is 37ºC (98.6ºF), but anywhere between 36.5ºC and 37.2ºC (97.7ºF and 99ºF) may be normal. Normal armpit temperatures are 0.2ºC to 0.3ºC lower than this.

A temperature of 38ºC (100.4ºF) or above is usually considered to be a significant fever - you should measure it again after two to three hours.

Fever is caused by the release of certain chemicals by the immune system, usually as a result of infection or inflammation.

In the past, many infections were fatal and fever was greatly feared, whether due to measles, flu or after childbirth. These days there are effective treatments for most infections. Even so, fever is an important sign that someone is ill and a cause should always be established.

Other causes include any condition where there's inflammation, from arthritis to trauma to inflammatory bowel disease.

Rare tropical haemorrhagic fevers include ebola, a highly infectious virus that rapidly breaks down the walls of arteries so the victim haemorrhages to death. It's fatal in 90 per cent of cases.

Such fevers are extremely rare, but there's a real risk of feverish infections such as malaria if you travel abroad, so seek advice before you travel.

When a doctor can't find the cause of a persistent fever it's called a fever of unknown origin (FUO, or PUO for 'pyrexia of unknown origin'). Special tests may be needed to pinpoint the cause and detect hidden abscesses or unusual infections.

About 25 per cent of FUOs aren't because of infection, so other explanations must be sought - most importantly cancer. About one in five people with chronic persistent fever has a tumour. People with lymphoma, lung, pancreatic or primary liver cancer often have fever caused by dead cancer cells or bleeding.

Not every fever needs medical attention, but in young children, if the temperature continues to rise or if there are other worrying symptoms, you may want to talk to your doctor.

There are several things you can do to help bring the temperature down and make the person more comfortable:

• Keep the room at a comfortable temperature, but make sure fresh air is circulating
• Ensure they drink plenty of water to prevent dehydration
• Give ice cubes to suck
• Take off excessive layers of clothing - small babies or children may be left in a thin vest

Give medication regularly. Either paracetamol as tablets (adults) or syrup (children); ibuprofen as tablets (adults) or syrup (children); or aspirin, for adults only. Children under 16 should not be given aspirin.

Fever is rarely harmful, but at very high temperatures it can cause problems. For example, some small children with a high fever will have a seizure, known as a febrile convulsion.

A high fever may also be a sign of serious illness. Call your doctor if you're at all worried, especially if:

• You suspect an infection, which might need antibiotic treatment
• It's in a very young child
• The child becomes unusually sleepy or doesn't respond
• The fever is higher than 38.5ºC (101.3ºF)
• The child won't take fluids
• A child or adult has a stiff neck, an unusual rash or difficulty breathing

This article was last medically reviewed by Dr Rob Hicks in April 2008