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A person has a fever if their body temperature rises above the normal range of 98–100°F (36–37°C). It is a common sign of an infection.
As a person’s body temperature increases, they may feel cold until it levels off and stops rising. People describe this as “chills.”
Eating, exercise, sleeping, the time of day, and individual factors can also affect temperature.
When an infection occurs, the immune system will launch an attack to try to remove the cause. A high body temperature is a normal part of this reaction.
A fever will usually resolve on its own. However, if body temperature rises too high, it may be a symptom of a severe infection that needs medical treatment. In this case, a doctor may recommend medication to reduce it.
Read on to learn more about the symptoms of a fever, as well as some causes and treatment options.
When someone has a fever, they may also:
- shiver and feel cold when nobody else does
- have a low appetite
- show signs of dehydration
- have increased sensitivity to pain
- lack energy and feel sleepy
- have difficulty concentrating
If a baby has a fever, they may:
- feel hot to the touch
- have flushed cheeks
- be sweaty or clammy
With a high fever, there may also be irritability, confusion, delirium, and seizures.
If a person has a fever with a dry cough, they may have symptoms of COVID-19.
The Centers for Disease Control and Prevention (CDC) urge people with these symptoms to stay at home and away from other people. The person should also wear a face covering if other people are near.
In many cases, the symptoms will improve without specialist treatment.
However, if the person also develops severe chest pain or difficulty breathing, they should call 911 and ask for medical help.
To take a temperature, most people now use a digital thermometer. Experts do not recommend using a glass thermometer, as these can be dangerous. Some people use a forehead strip, but these may be less accurate.
A person can put a thermometer under their arm or in their mouth.
- Clean the tip using cold water and soap, and then rinse it.
- Turn on the device.
- Place the tip under the tongue, toward the back of the mouth, and close the mouth. Or, place it under the armpit and hold the device close to the body.
- Wait until there is a flash or the thermometer beeps.
- Read the temperature.
Normal armpit temperature will be around 0.5 to 0.9ºF (0.3 to 0.5°C) lower than oral temperature.
If the reading is 100.4°F (38°C) or above, the person has a fever.
Is it possible to take someone’s temperature without a thermometer? Find out here.
Doctors classify fevers according to how long they last, whether or not they come and go, and how high they are.
Core body temperature varies from person to person.
Most experts consider a temperature of 100.4° F (38°C) to be a fever, but in children, this may be lower, at 99.5°F (37.5°C).
A fever can be:
- acute if it lasts for under 7 days
- subacute if it lasts for up to 14 days
- chronic or persistent if it lasts for over 14 days
Fevers that exist for days or weeks with no explanation are called fevers of unknown origin.
A mild fever is part of the immune system’s response to bacteria, viruses, and other pathogens. It helps the body fight off infection.
However, it can be uncomfortable, and a high fever can sometimes lead to complications.
For this reason, doctors may sometimes recommend medications called antipyretics to lower a person’s temperature.
Examples include nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen. Acetaminophen (Tylenol) can also reduce a fever. Aspirin can help, but it is not suitable for children, and it may not be suitable for people who take blood thinners.
If a person is sweating a lot, they may experience dehydration. In this case, they should consume plenty of fluids to prevent complications.
Treating the cause
A fever is a symptom, not an illness.
A doctor may wish to carry out tests to identify the cause. If the fever is due to a bacterial infection, they may prescribe an antibiotic.
If it stems from a viral infection, the doctor may recommend using NSAIDs to relieve the symptoms.
Antibiotics will not stop a virus. A doctor will not prescribe them for a viral infection.
NSAIDs will not help if the fever is due to hot weather or sustained strenuous exercise. In these cases, it is essential to cool the person down. If they are confused or unconscious, they need immediate medical care.
A person with symptoms of COVID-19 may not need any medical treatment.
However, if they develop severe chest pains and have difficulty breathing, they may need hospital treatment.
Some people will need to spend time on a ventilator, which is a device that will help them breathe.
Fevers can result from various factors, including:
- an infection, such as strep throat, the flu, chickenpox, pneumonia, or COVID-19
- rheumatoid arthritis
- some medications
- overexposing the skin to sunlight, or sunburn
- heatstroke, either due to high ambient temperatures or prolonged strenuous exercise
- silicosis, which is a type of lung disease caused by long-term exposure to silica dust
- amphetamine abuse
- alcohol withdrawal
These often result from an ear infection, gastroenteritis, or a respiratory virus, and they are not usually serious. Less commonly, they may stem from a more severe illness, such as meningitis, a kidney infection, or pneumonia.
Seizures can occur when the body temperature rises quickly.
There are two types of febrile seizure: simple febrile seizures and complex febrile seizures.
Simple febrile seizures
This type of febrile seizure can last from a few seconds up to 15 minutes. However, it usually lasts less than 5 minutes. It does not occur again during a 24-hour period.
Around 80–85% of febrile seizures are of this type.
It typically involves the whole body, and symptoms include:
- stiffness in the body
- twitching in the arms and legs
- a loss of consciousness while the eyes stay open
There may also be:
- irregular breathing
- urination, defecation, or both
Complex febrile seizures
This type of febrile seizure lasts longer than 15 minutes, comes back more often, and tends to affect only a part of the body, rather than the whole body.
Complex febrile seizures are more serious than simple febrile seizures.
A child who has a complex febrile seizure is more likely to experience epilepsy as they grow older.
In fact, around 30–40% of children who have either type of seizure will have other similar seizures at a later date.
When to see a doctor
In most cases, a child who has a seizure should see a doctor. The doctor may suggest controlling their temperature with acetaminophen and ensuring that they drink plenty of fluids.
If necessary, they may also prescribe an anticonvulsant, such as sodium valproate or clonazepam.
A fever is a symptom, not an illness. A doctor can diagnose a fever by checking the person’s body temperature, but they will also need to diagnose the reason for the fever.
To do so, they will examine the individual and ask them about any other symptoms and their medical history.
If the person has recently experienced another infection, if they have recently had surgery, or if there is pain or swelling in one area, it may indicate what kind of infection is likely to be present.
To confirm a diagnosis, the doctor may recommend:
- a blood test
- a urine test
- imaging tests
The treatment they prescribe will depend on the cause of the fever.
To prevent a fever, people should follow the usual steps for reducing the risk of an infection.
These include regular hand-washing and staying away from people who are unwell.
To reduce the risk of transmitting or contracting COVID-19, the CDC recommend:
- washing the hands regularly with soap and water for at least 20 seconds each time
- using hand sanitizer that contains at least 60% alcohol if soap and water are not available
- not touching the face with unwashed hands
- covering the face with a mask or cloth covering when near other people, except for children under 2 years
- cleaning and disinfecting surfaces regularly
- coughing and sneezing into a tissue, then disposing of the tissue and washing the hands
- avoiding close contact with people who are unwell
A fever is usually a symptom of an infection. It is not usually a cause for concern, but the underlying illness may need medical treatment.
Often, a fever will resolve without medical attention. However, if a child or an older adult has a fever, if the individual has other severe or worsening symptoms, or if they have a weakened immune system, they should seek medical help.
If a person has a dry cough with a fever, they may have COVID-19. If they start to experience difficulty breathing, someone should call 911 and ask for emergency help.