A fever is a temporary rise in body temperature. It's one part of an overall response from the body's immune system. A fever is usually caused by an infection.

For most children and adults, a fever may be uncomfortable. But it usually isn't a cause for concern. For infants, however, even a low fever may mean there's a serious infection.

Fevers generally go away within a few days. A number of over-the-counter medications lower a fever. But you don't necessarily need to treat a fever if it's not causing discomfort.

Body temperatures vary slightly from person to person and at different times of day. The average temperature has traditionally been defined as 98.6 F (37 C). A temperature taken using a mouth thermometer (oral temperature) that's 100 F (37.8 C) or higher is generally considered to be a fever.

Depending on what's causing a fever, other fever signs and symptoms may include:

• Sweating
• Chills and shivering
• Headache
• Muscle aches
• Loss of appetite
• Irritability
• Dehydration
• General weakness

Taking a temperature

To take a temperature, you can choose from several types of thermometers, including oral, rectal, ear (tympanic) and forehead (temporal artery) thermometers.

Oral and rectal thermometers generally provide the most accurate measurement of core body temperature. Ear or forehead thermometers, although convenient, provide less accurate temperature measurements.

In infants, a rectal temperature, if doable, is somewhat more accurate. When reporting a temperature to your health care provider, give both the reading and the type of thermometer used.

Fevers by themselves may not be a cause for alarm — or a reason to call a doctor. Yet there are some circumstances when you should seek medical advice for your baby, your child or yourself.

Infants and toddlers

A fever is a particular cause for concern in infants and toddlers. Call your baby's health care provider if your child is:

• Younger than 3 months old and has a rectal temperature of 100.4 F (38 C) or higher.
• Between 3 and 6 months old and has a rectal temperature higher than 102 F (38.9 C) or has a lower temperature but seems unusually irritable, sluggish or uncomfortable.
• Between 7 and 24 months old and has a rectal temperature higher than 102 F (38.9 C) that lasts longer than one day but shows no other symptoms. If your child also has other signs and symptoms, such as a runny nose, cough or diarrhea, you can call sooner.

Children

There's probably no cause for alarm if your child has a fever but is responsive. This means your child makes eye contact with you and responds to your facial expressions and to your voice. Your child may also be drinking fluids and playing.

Call your child's health care provider if your child:

• Is listless, confused or has poor eye contact with you.
• Is irritable, vomits repeatedly, has a severe headache, sore throat, stomachache or other symptoms causing a lot of discomfort.
• Has a fever after being left in a hot car. Seek medical care immediately.
• Has a fever that lasts longer than three days.
• Has a seizure associated with the fever. Call 911 if the seizure lasts more than five minutes or your child doesn't recover quickly.

Ask your child's health care provider for guidance in special circumstances, such as a child with immune system problems or with a preexisting illness.

Adults

Call your health care provider if your temperature is 103 F (39.4 C) or higher. Seek immediate medical attention if any of these signs or symptoms accompanies a fever:

• Severe headache
• Rash
• Unusual sensitivity to bright light
• Stiff neck and pain when you bend your head forward
• Mental confusion, strange behavior or altered speech
• Persistent vomiting
• Difficulty breathing or chest pain
• Abdominal pain
• Pain when urinating
• Convulsions or seizures

Typical body temperature is a balance of heat production and heat loss. An area in the brain called the hypothalamus (hi-poe-THAL-uh-muhs) — also known as your body's "thermostat" — monitors this balance. Even when you're healthy, your body temperature varies slightly throughout the day. It can be lower in the morning and higher in the late afternoon and evening.

When your immune system responds to disease, the hypothalamus can set your body temperature higher. This prompts complex processes that produce more heat and restrict heat loss. The shivering you might experience is one way the body produces heat. When you wrap up in a blanket because you feel chilled, you are helping your body retain heat.

Fevers below 104 F (40 C) associated with common viral infections, such as the flu, may help the immune system fight disease and are generally not harmful.

Fever or elevated body temperature might be caused by:

• A viral infection
• A bacterial infection
• Heat exhaustion
• Certain inflammatory conditions such as rheumatoid arthritis — inflammation of the lining of your joints (synovium)
• A cancerous (malignant) tumor
• Some medications, such as antibiotics and drugs used to treat high blood pressure or seizures
• Some immunizations, such as the diphtheria, tetanus and acellular pertussis (DTaP), pneumococcal or COVID vaccine

Children between the ages of 6 months and 5 years are at increased risk of a seizure that occurs during a fever (febrile seizure). About a third of the children who have one febrile seizure will have another one, most commonly within the next 12 months.

A febrile seizure may involve loss of consciousness, shaking of limbs on both sides of the body, eyes rolling back or body stiffness. Although alarming for parents, the vast majority of febrile seizures cause no lasting effects.

If a seizure occurs:

• Lay your child on the side or stomach on the floor or ground
• Remove any sharp objects that are near your child
• Loosen tight clothing
• Hold your child to prevent injury
• Don't place anything in your child's mouth or try to stop the seizure
• Call 911 or your local emergency number if a seizure lasts more than five minutes or your child doesn't appear to recover well after the seizure
• Get emergency room or urgent care services if it's your child's first febrile seizure.

If your child doesn't need emergency care, see your child's health care provider as soon as possible for further evaluation.

You may be able to prevent fevers by reducing exposure to infectious diseases. Here are some tips that can help:

• Get vaccinated as recommended for infectious diseases, such as influenza and COVID-19.
• Follow public health guidelines for wearing masks and social distancing.
• Wash your hands often and teach your children to do the same, especially before eating, after using the toilet, after spending time in a crowd or around someone who's sick, after petting animals, and during travel on public transportation.
• Show your children how to wash their hands thoroughly, covering both the front and back of each hand with soap and rinsing completely under running water.
• Carry hand sanitizer with you for times when you don't have access to soap and water.
• Try to avoid touching your nose, mouth or eyes, as these are the main ways that viruses and bacteria can enter your body and cause infection.
• Cover your mouth when you cough and your nose when you sneeze, and teach your children to do the same. Whenever possible, turn away from others and cough or sneeze into your elbow to avoid passing germs along to them.
• Avoid sharing cups, water bottles and utensils with your child or children.

To evaluate a fever, your care provider may:

• Ask questions about your symptoms and medical history
• Perform a physical exam
• Take nasal or throat samples to test for respiratory infections
• Order tests, such as blood tests or a chest X-ray, as needed, based on your medical history and physical exam

Because a fever can indicate a serious illness in a young infant, especially two months of age or younger, your baby might be admitted to the hospital for testing and treatment.

Fever of unknown origin

When a fever lasts for more than three weeks — constantly or on several occasions — and there is no clear cause, it's usually called a fever of unknown origin. In these cases, you may need to see specialists in one or more medical fields for further evaluations and tests.

For a low-grade fever, your care provider may not recommend taking medications to lower your body temperature. These minor fevers may be helpful in reducing the number of microbes causing your illness. Fevers above 102 F (38.9 C) tend to cause discomfort and often require treatment.

Over-the-counter medications

In the case of a high fever or a fever that causes discomfort, your care provider may recommend nonprescription medication, such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others).

Use these medications according to the label instructions or as recommended by your health care provider. Be careful not to take too much. High doses or long-term use of acetaminophen or ibuprofen may cause liver or kidney damage, and acute overdoses can be fatal. Don't give aspirin to children, because it may trigger a rare, but potentially fatal, disorder known as Reye's syndrome.

These medications will usually lower your temperature, but you may still have a mild fever. It may take 1 to 2 hours for the medication to work. Call your care provider if your fever doesn't improve, even after taking medication.

Prescription medications

Your health care provider may prescribe other medications based on the cause of your illness. Treating the underlying cause may lessen signs and symptoms, including fever.

Treatment of infants

Infants, especially those younger than two months old, might need to be admitted to the hospital for testing and treatment. In babies this young, a fever could indicate a serious infection that requires intravenous (IV) medications and round-the-clock monitoring.

You can try a number of things to make yourself or your child more comfortable during a fever:

• Drink plenty of fluids. Drinking fluids will improve heat loss from the skin and replace water lost through sweating. Water and clear broth are healthy choices. Infants under 6 months should only have breast milk or formula.
• Rest. You need rest to recover, and activity can raise your body temperature.
• Stay cool. If you aren't shivering, dress in light clothing, keep the room temperature cool, and sleep with only a sheet or light blanket.

Your appointment may be with your family doctor, pediatrician or other care provider. Here's some information to help you get ready for your appointment and know what to expect from your care provider.

What you can do

• Be aware of any pre-appointment restrictions. When you make the appointment, ask if there's anything you need to do in advance.
• Write down information about the fever, such as when it started, how and where you measured it (orally or rectally, for example) and any other symptoms. Note whether you or your child has been around anyone who's been ill.
• Write down key personal information, including possible exposure to anyone who's been ill or recent travel out of the country.
• Make a list of all medications, vitamins and supplements that you or your child is taking.
• Write down questions to ask the care provider.

For a fever, some basic questions to ask your provider include:

• What's likely causing the fever?
• What kinds of tests are needed?
• What treatment approach do you recommend?
• Is medicine necessary to lower the fever?
• Are there any restrictions that I need to follow?

Don't hesitate to ask other questions during your appointment as they occur to you.

What to expect from your doctor

Be prepared to answer questions, such as:

• When did the symptoms first occur?
• What method did you use to take your or your child's temperature?
• What was the temperature of the environment surrounding you or your child?
• Have you or your child taken any fever-lowering medication?
• What other symptoms are you or your child experiencing? How severe are they?
• Do you or your child have any chronic health conditions?
• What medications do you or your child regularly take?
• Have you or your child been around anyone who's ill?
• Have you or your child recently had surgery?
• Have you or your child recently traveled outside the country?
• What, if anything, seems to improve the symptoms?
• What, if anything, appears to worsen the symptoms?