Fever facts

Although a fever - also known as a high fever or a high temperature - is not by itself an illness, it's usually a symptom of an underlying condition, most often an infection.

Fever is usually associated with physical discomfort and most people feel better when a fever is treated. But depending on your age, physical condition, and the underlying cause of your fever, you may or may not require medical treatment for the fever alone. Many experts believe that fever is a natural bodily defence against infection. There are also many non-infectious causes of fever.

Fever is generally not considered dangerous, but hyperthermia is associated with dangerous rises in body temperature. This can be due to an extreme temperature associated with heat injury such as heat stroke, side effects of certain medications or illicit drugs and stroke. With hyperthermia, the body is no longer able to control body temperature.

Children and fever

In children with fever, accompanying symptoms such as lethargy, irritability, poor appetite, sore throat, cough, ear pain and diarrhoea are important to relay to your doctor.

For children with fevers, NHS Choices advises:

Measured under the arm, normal temperature is about 36.4°C (97.4°F). Under the tongue, normal temperature is slightly higher at about 37°C (98.4°F). This may vary a bit.

If you're worried speak to your GP or call NHS Direct on 0845 4647, or NHS24 in Scotland on 08454 242424. If the surgery is closed contact your GP out-of-hours service. If you're still concerned, or if your GP or out-of-hours service can’t respond quickly enough, take your child straight to the Accident and Emergency (A&E) department of your nearest hospital.

Always contact your GP, health visitor, practice nurse or nurse practitioner if:

• your child has other signs of illness as well as a raised temperature,
• your baby’s temperature is 38°C (101°F) or higher (if they’re under three months), or

your baby’s temperature is 39°C (102°F) or higher (if they’re three to six months).

Causes of fever

A part of the brain called the hypothalamus controls body temperature, which usually varies throughout the day from the considered normal temperature of 37°C (98.6 F). Normal body temperature is generally a little cooler in the early morning but may reach as high as 37.7°C (99.9 F) by late afternoon.

In response to an infection, illness, or some other cause, the hypothalamus may reset the body to a higher temperature. The substances that cause fever are called pyrogens, which can come from inside the body (proteins called cytokines) or outside of the body (such as germs or their toxins).

Although the most common causes of fever are common infections such as colds and gastroenteritis, other causes include:

• Infections of the ear, lung, skin, throat, bladder or kidney
• Inflammatory conditions
• Side effects of drugs
• Cancer
• Routine vaccinations

Other causes of fever include:

• Blood clot or pulmonary embolism
• Autoimmune diseases such as lupus, rheumatoid arthritis and inflammatory bowel disease
• Hormone disorders such as hyperthyroidism
• Illegal drugs that increase heat production such as amphetamines and cocaine