Your body is burning up with heat, you probably don't feel well, and you may be shivering too, so what does it all mean?
Fever is a body temperature that is higher than normal, which ranges between 36.5 and 37.4 degrees Celsius. It is generally seen as a symptom of an underlying disease, rather than an illness itself.
There's obviously a lot going on, so to explain more, here we'll look at:
Your metabolism works best when your temperature is within the normal range and it's usually kept in check thanks to your body's 'thermostat' — an area at the base of your brain, known as the hypothalamus.
However, if you're infected with a bacteria, a virus or another germ, your body releases chemicals known as pyrogens that stimulate the hypothalamus, causing physiological changes in your body that result in a fever.
Definitions of fever vary but generally, a temperature above 38C is considered abnormal.
A mild fever (up to 39C) can be beneficial as it makes your body a less favourable host for viruses and bacteria (they are temperature sensitive).
It also helps stimulate your immune system, making it more effective at fighting these pathogens. For this reason treatment of fever is not always necessary or beneficial.
What causes a fever?
Normally a fever is the result of an infection, but there are other occasions where it may arise, although most are relatively rare:
- After severe sunburn
- After an immunisation
- Drug reactions
- Inflammatory conditions such as rheumatoid arthritis
- Following major trauma or surgery
- In certain cancers
What are the symptoms of fever?
A fever can be short-lasting (acute) or ongoing (chronic), or it can be intermittent as is often the case in malaria, for example.
Elderly people tend to get lower fevers than younger people do in the same circumstances however newborn infants, especially those born prematurely, may have a drop in temperature when they have an infection.
A fever in itself is usually not dangerous unless it becomes very high and an internal body temperature in excess of 42C, for instance, can cause damage to vital organs.
In children, a high fever — especially if it has risen very quickly — can cause fits, called febrile convulsions. These are not usually harmful but if you're a parent, it can be frightening to witness.
Other common fever symptoms include:
- Feeling unwell
- Shivering or shaking (usually occurs when you have a rising fever)
- Sweating (this usually occurs when the fever subsides)
- Flushed face
- Headache and muscle aches
How do you diagnose and treat a fever?
Fever itself isn't a disease but rather a symptom of an underlying condition, so if a doctor examines you (when you have a fever) they are looking for the disease that is causing it.
Your doctor will usually take a history and do a physical examination. Tests may be ordered to try to find the source of any infection.
If you have a bacterial infection, your doctor may prescribe a course of antibiotics but if you have a viral infection, antibiotics won't be prescribed because they have no effect against viruses.
In many cases, no specific treatment is needed and you'll get better with rest and plenty of fluids.
Simple over-the-counter, fever-reducing medications, such as aspirin or paracetamol for adults, or paracetamol or ibuprofen for children, will help bring down a fever.
These will only provide relief from symptoms and will not treat the underlying infection or other cause.
Aspirin shouldn't be given to infants or children under the age of 12 years as it may cause a rare but serious disorder known as Reye's syndrome.
When should your child see a doctor?
Most fevers in children result from viral illnesses that get better without treatment.
Immunising your child in time also protects them from a variety of more serious infections.
For children, you should seek medical attention if they have a fever and:
- Are under six months of age
- Have a fever of 40C or more with no other symptoms
- Won't eat or drink
- Have a headache, earache, or stiff neck
- Have a rash or can't tolerate bright lights
- Are passing less urine than normal
- Are very drowsy, sleepy, confused, or can't be woken
- Have a seizure (convulsion)
- Vomit or have persistent diarrhoea
- Cry continually and won't be comforted
- Have difficulty breathing, including breathing more rapidly
- Are in pain
What should you do if your child has a convulsion?
In some children (about one in 30) a high fever can cause febrile convulsions or 'fever fits' at one time or another.
A child may experience sudden convulsions and or loss of consciousness.
Their muscles may stiffen and jerk, and they may go red or blue in the face. This may last for several minutes.
Although distressing for parents, these convulsions do not cause brain damage and almost never cause any lasting effects.
There's nothing you can do to stop a convulsion.
You should try and stay calm, put your child on a soft surface, lying on their back or side, and watch what happens so you can describe it later.
Time how long the convulsion lasts.
If the convulsions last more than five minutes, your child doesn't wake up when the convulsion stops, or your child looks very sick when the convulsion stops, call triple-0.
Otherwise make an appointment with your family doctor to discuss the episode.
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Fevers in adults
Adults with a fever should see a doctor if:
- The fever persists after three days, despite simple home-based measures
- The fever is high (over 40C)
- They are shivering and shaking involuntarily
- They have a severe headache (especially if it's not responding to painkillers) or become drowsy or confused
- They have recently travelled overseas
If you think a child or adult with a fever may need an ambulance, but are unsure, you can call triple-0 and speak to someone.
If you just want advice on a health condition and what to do next, healthdirect Australia 1800 022 222 is a free 24-hour telephone line staffed by registered nurses.
This is general information only. For detailed personal advice, you should see a qualified medical practitioner who knows your medical history.
This story, which was originally reviewed by David Burgner of the Murdoch Children's Research Institute and published by ABC Health and Wellbeing, was updated in 2019.