Fever

Also known as a high temperature

A fever is when your core temperature is raised above 38°C. It often accompanies an infection, such as a cold or flu.

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

Normal body temperature for children and adults is around 37ºC (degrees Celsius). A fever is when your core body temperature is higher than 38ºC (degrees Celsius). Usually, you will feel unwell and hot, and you may sweat. Sometimes you will feel very cold and shivery even when your temperature is high.

Mild fever: You have a mild fever if your temperature is between 38ºC and 38.9ºC.
High fever: A high fever is when your temperature is between 39ºC and 39.9ºC.
Very high fever: A very high fever is more than 40ºC or higher.

You can use a thermometer to find out how high your temperature is. Read more about thermometers and how to use them.


COVID-19 symptoms

A fever, with or without respiratory symptoms such as a cough or runny nose, could also be a COVID infection. Test for COVID-19 and stay home until well or sure this isn’t COVID.

Image credit: Canva

Looking after yourself when you have a fever

A fever usually lasts 3–5 days and most people recover from a mild fever by managing their symptoms at home. However, there are times when you should see a doctor urgently for fever

  • Drink plenty of fluids, especially water or rehydration fluid (little and often is best). You sweat more when you have a fever. Drink enough so your urine (pee) is light yellow and clear.
  • Rest while you recover.
  • Open a window for ventilation but avoid draughts.
  • Wear lightweight comfortable clothing and use lighter bedding. Don't use hot water bottles or electric blankets.
  • Use a cool cloth to wash your face, hands and neck.
  • Change bed linen and clothing regularly, especially if they are wet from sweat. 

When to seek help

Although most fevers will settle down in a few days and are not worrying, you sometimes need to seek medical advice.

Contact a healthcare provider or call Healthline 0800 611 116 for advice: 

If you have a fever and the following symptoms:

  • a very high fever (40ºC or over)
  • you are passing urine that is darker than normal
  • you are still feverish after 3 days or seem to be getting sicker
  • you are shivering or shaking uncontrollably, or have chattering teeth, and it doesn’t stop within an hour or so
  • you have a severe headache that doesn’t get better after taking pain medicines
  • are having trouble breathing
  • are getting confused or are unusually drowsy
  • have recently travelled overseas.

If you have a fever and you are:

  • being treated for immune deficiency
  • on immune-suppressant drugs, such as regular steroids, methotrexate, azathioprine or cyclophosphamide
  • taking medication where you have been warned about a risk of a reduced immune system
  • having, or have recently completed, treatment for cancer, leukaemia or lymphoma
  • a transplant recipient
  • HIV positive.

If you’re pregnant and have a fever, check with your midwife, doctor or nurse before you take any medicines. If your fever lasts for longer than a day, talk to your lead maternity carer (LMC).

Call 111 and ask for an ambulance or go to the nearest hospital

If you have a fever and any of the following symptoms: 

  • Hallucinations.
  • Vomiting.
  • A stiff neck (unable to put your chin on your chest or have pain when moving your neck forward).
  • A skin rash.
  • A rapid heart rate.
  • A seizure (fit), or experience signs of a seizure about to happen, such as regular twitching or jerking.

What causes fever?

A viral infection is the most common cause of a fever. A bacterial infection is less common but is more serious. Your body's natural reaction to infection is to raise your body temperature. This helps kill the infection. Vaccination sometimes causes a mild fever too.

Do I need medicine for fever?

Fever is your body’s way of fighting infection. Medicine is not needed for mild fever, but you can use paracetamol if you also have a headache or muscle pain. 

Learn more

Fever in adults  Ministry of Health, NZ
Influenza Ministry of Health, NZ

References

  1. Fever and night sweats Patient Info, UK, 2015
  2. Cold season – managing without antibiotics BPAC, NZ, 2018
  3. Danger signs during pregnancy Ministry of Health, NZ, 2017
  4. Fever in adults NHS Inform, UK, 2020

See also Clinicians page on fever in children.

Information for healthcare providers

Fever and night sweats Patient Info Professional, UK, 2016
Cold season – managing without antibiotics BPAC, NZ, 2018

Reviewed by

Dr Sharon Leitch is a general practitioner and Senior Lecturer in the Department of General Practice and Rural Health at the University of Otago. Her area of research is patient safety in primary care and safe medicine use.
Credits: Health Navigator Editorial Team. Reviewed By: Dr Sharon Leitch, GP and Senior Lecturer, University of Otago Last reviewed: 18 Sep 2020