This page provides general information about fever. If you have a child with a fever see our page on fever in children.

What causes fever?

Fever is usually caused by a viral or bacterial infection, although immunisation can sometimes cause mild fever in both adults and children.

Fever is the body's way of fighting infection. Raising the temperature inside the body helps to kill the virus or bacteria causing the infection.

Taking someone's temperature using a digital thermometer

Digital thermometers are easy to read and can be used in all age groups. Young children can have their temperature measured in the armpit, and older children and adults in the mouth.

Follow the instructions that come with the thermometer. They need to be turned on and some will beep while measuring and some will beep when finished reading. Aim to keep the thermometer in place for two minutes, as a general guide.

What do the numbers on the thermometer mean?

The numbers on the thermometer show what a person's temperature is: 

• Normal body temperature is around 37 degrees Celsius.
• A mild fever is above 38ºC
• A high fever if it is 39ºC and above.

A fever by itself does not mean that someone is seriously unwell – the important thing is to check for other signs of illness.

Do I need to see a doctor about fever?

Young babies/pregnant women

Babies under three months old with fever should be checked by a doctor. Pregnant women with fever should contact their lead maternity carer or doctor.

Other ages

Older infants, children and adults will not usually need any medical treatment for mild fever, especially if they are otherwise well. However, if the fever occurs in addition to other symptoms listed in box at this top of the page and the person is unwell, it is important to get urgent medical advice.

Also see your doctor if the fever gets worse, doesn’t go away in a couple of days or is associated with other symptoms such as:

• sore throat or joint pain
• frequent, watery poo (diarrhoea)
• pain when weeing
• becoming unwell after recent overseas travel.

Check with your doctor or phone Healthline on 0800 611 116 if you are worried. If you have already seen a doctor but symptoms worsen, you should go back for another check.

Do I need medication for fever?

Fever is your bodies way of fighting infection. Medication is not needed for mild fever, but paracetamol can be used if there is also headache, pain or distress.

• Use correct doses as listed on the product and measure children’s doses accurately.
• If you have been given ibuprofen for your child, don't use it as well as paracetamol. Use one or the other and only if your child is miserable or complaining of headache or other pain.
• Cold and flu medicines are not recommended for babies and children.
• Do not give aspirin to children under 16 years as it can cause Reye’s syndrome.
• If unsure of any product, check with your local pharmacist.

Fever and vaccinations

A mild fever may be experienced after receiving a vaccination; however, it is not recommended to use paracetamol routinely before or after a vaccination as it may make the vaccination less effective. Ask your doctor or practice nurse for more information.

How to care for someone with fever

• make sure their room temperature is comfortable (not too hot or too cold)
• if possible, open a window for ventilation but avoid draughts
• keep the person in lightweight clothing and bedding
• rest is important as the fever means they are fighting an infection
• do not use hot water bottles or electric blankets during fever
• make sure the person takes plenty of fluids (little and often is best)
• a lukewarm flannel-bath can be refreshing – using a lukewarm flannel, wash the face, hands and neck
• change the bed linen and clothing regularly.

Learn more

Fever in children Healthed (NZ), July 2014
Fever in adults Healthed (NZ), Dec 2011
Fever Kidshealth (NZ), Nov 2012
How to take a temperature Kidshealth (NZ), Sept 2013