Fever in adults

This page has general information about fever in adults. If you have a child with a fever, see the page on fever in children.

Normal body temperature is usually between 36 and 36.8 degrees Celsius.

A high temperature or fever is when your body temperature is 38 degrees Celsius or higher. It can be a sign that you are unwell. It usually means you have an infection, such as a cold. But it can also be due to more serious infections, such as COVID-19 (coronavirus).

Symptoms of COVID-19 and what to do

A fever is usually caused by your body fighting a viral or bacterial infection. It usually lasts around 3 to 5 days. Most people recover from a mild fever with self-care at home.

Symptoms

Your symptoms will depend on the type of fever you have.

Mild fever (38 to 38.9 degrees Celsius)

With a mild fever, you might:

• have flushed cheeks
• feel tired
• be warm to the touch

You will usually be able to carry out normal daily activities.

High fever (39 to 39.9 degrees Celsius)

With a high fever, you might:

• feel hot to the touch
• not feel well enough to go to work
• have aches and pains

Very high fever (40 degrees Celsius or higher)

With a very high fever, you will usually want to stay in bed or be inactive. You will not feel well enough to carry out normal activities. You’ll feel hot to the touch and you may have lost your appetite.

When to see a GP

Most older infants, children and adults do not need medical treatment for a mild fever.

You should contact your GP if you:

• have a very high fever (40 degrees Celsius or higher)
• are still feverish after 3 days of home treatment or seem to be getting sicker
• are shivering or cannot stop shaking, or have chattering teeth, and it does not stop within an hour or so
• have a severe headache that does not get better after taking painkillers
• are having trouble breathing
• are getting confused or are very drowsy
• have travelled overseas recently

Young babies and pregnant women

Babies under 3 months with a fever must see a GP. If your baby is between 3 and 6 months and has a high or very high fever, contact your GP.

Fever in babies and children

If you are pregnant and have a temperature of 38.5 degrees Celsius, or any fever lasting for 3 days or more, see your GP. They need to monitor the effects of the fever on your baby.

Causes of fever

Fever is usually caused by a viral or bacterial infection. Fever is your body's way of fighting infection. Raising your body temperature helps to kill the virus or bacteria.

Common conditions that can cause fevers include:

• respiratory tract infections
• flu (influenza) and flu-like conditions
• colds
• tummy bugs (gastroenteritis)
• ear infections
• infection of your tonsils (tonsillitis)
• kidney or urinary tract infections

Common childhood illnesses that can cause fevers include:

• chickenpox
• scarlet fever
• rubella (German measles)
• whooping cough
• rheumatic fever

You might have a mild fever after getting a vaccine. Adults should not take paracetamol before or after getting a vaccine. It may make the vaccine less effective.

Treatment

Medicine is not needed for a mild fever. You can use paracetamol if you also have a headache, pain or distress.

Managing a fever yourself

Drink plenty of fluids, especially water (little and often is best). Try to drink 2 to 3 litres in 24 hours while you have a fever. This will reduce the risk of dehydration.

Content supplied by the NHS and adapted for Ireland by the HSE