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A fever is when your body temperature is higher than your normal average temp. Most providers say a fever is either 100.0 F (37.8 C) or 100.4 F (38 C). When you have a fever, it’s your body’s normal response to fighting an infection or illness. Fevers usually aren’t a serious concern. They typically go away when the infection passes.
A fever is when your body temperature rises higher than normal. A fever itself isn’t an illness. Rather, it’s a symptom of a wide range of health conditions. When your body temperature rises a few degrees above normal, it can be a sign that your immune system has been activated, often to fight an infection. It can also be a side effect of some medications and vaccinations.
Common knowledge states that a “normal” body temperature is 98.6 degrees Fahrenheit (37 degrees Celsius). But your baseline body temperature may vary by a degree or more. It also fluctuates. It’s typically lower in the morning and higher in the evening. It’s higher during certain points of your menstrual cycle and when you’re exercising.
Infants and young children normally have slightly higher body temperatures than older children and adults. So infants and younger children have slightly higher fever temperatures.
There are no strict guidelines for what temperature is a fever, because body temperature varies by person, time of day and method of measurement. But healthcare providers most often say a fever temperature is 100.4 degrees F (38 degrees C) when measured orally (by mouth).
Rectal and ear thermometers typically measure temperatures at about 1.0 degrees F (0.6 degrees C) higher than oral thermometers. Skin thermometers (such as forehead thermometers) typically measure temperatures at about 1.0 degrees F (0.6 degrees C) lower than oral thermometers.
A low-grade fever means a body temperature slightly above normal. This type of fever can be a sign your immune system has been mildly activated. There’s no standard low-grade fever range. But many healthcare providers consider a body temperature between 99.5 degrees F (37.5 degrees C) and 100.3 degrees F (38.3 degrees C) to be a low-grade fever.
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In addition to an elevated temperature, you may experience the following symptoms:
Additional fever symptoms in babies and children may include:
A fever has many causes and can be a symptom of almost any illness. Common conditions that cause fever include:
You may also develop a fever due to:
Allergies can cause cold-like symptoms such as sneezing, runny nose and watery eyes, but they typically don’t cause fever. Despite its name, hay fever (also called allergic rhinitis) doesn’t cause fever.
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You can take your temperature in several different parts of your body. The most common site is your mouth (oral temperature). Other sites include your ear (tympanic membrane), forehead (temporal artery) and armpit (axillary). The most accurate site is the rectum, and the least accurate site is the armpit.
The best way to measure your temperature is with a digital thermometer. Glass thermometers contain mercury and providers no longer recommend them.
If your fever is mild — less than 101 degrees F (38.3 degrees C) — you usually don’t need treatment with medication. Make sure you drink plenty of fluids and get plenty of rest. Taking a lukewarm bath — about 98 degrees F (36.7 degrees C) — may also help bring your body’s temperature down. Keep an eye on signs of dehydration and other worsening symptoms.
If you’re feeling uncomfortable and your temperature is higher than 101 degrees F (38.3 degrees C), you can try to break the fever. The most common way to get rid of a fever is to use over-the-counter (OTC) medications such as acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs), including ibuprofen, naproxen and aspirin.
Don’t give aspirin to children under the age of 17. Aspirin in children may cause Reye’s syndrome, a sometimes fatal illness.
Yes, you can take nonsteroidal anti-inflammatory drugs such as ibuprofen to try to reduce a fever.
The cause of the fever determines how long it’ll last. Typically, if an infection is the cause of your fever, it should pass within three to four days. Call your healthcare provider if the fever lingers longer than that or comes along with breathing changes.
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You can prevent some fevers by avoiding the illnesses and infections that cause them. Sometimes, children develop fevers after receiving vaccinations. You can prevent these types of fevers by giving your child acetaminophen just before or after the vaccination. But you can’t prevent most fevers. They’re typically a sign that your body is doing what it’s supposed to do.
In adults, fevers less than 103 degrees F (39.4 degrees C) typically aren’t dangerous and aren’t a cause for concern. If your fever rises above that level, make a call to your healthcare provider for treatment.
In children, call your child’s healthcare provider if:
If you have a fever along with any of the following symptoms, go to your nearest emergency room or call 911. It may be a sign of a serious or life-threatening illness.
If your baby has a fever and is under 3 months of age, take them to the ER right away. Your baby’s immune system hasn’t developed fully yet, and they could have a serious infection.
If your child has a fever along with any of the following symptoms, call 911 or go to the nearest emergency room:
Untreated fevers above 105.8 degrees F (41 degrees C) can be dangerous. If your body temperature reaches this level, your organs will begin to malfunction and will eventually fail.
Even moderate fevers can be dangerous for adults with lung or heart disorders because fever causes your breathing rate and heart rate to increase. Fever can also worsen the mental state of people with dementia.
A recurrent fever is a fever that keeps coming back multiple times over a period of time. Providers call these fevers episodic because they come and go. The fever lasts for a few days and then goes away for a stretch of time. Your child is healthy and acts normally between fevers. Recurrent fevers affect young children under the age of 5 most often. They’re one of the main symptoms of a collection of conditions called periodic fever syndromes.
Some children have a side effect of fevers called febrile seizures. This happens in 2% to 4% of children under the age of 5. Some seizures cause jerking movements, or it may look like your child has passed out. When this happens, put your child on their side and don’t put anything in their mouth. Call 911 if the seizure lasts for more than five minutes and/or your child’s lips turn blue.
If the seizure lasts less than five minutes, notify their healthcare provider and get medical attention right away.
Fever dreams are vivid, bizarre or unpleasant dreams you may experience when your body temperature rises higher than normal. Researchers don’t know the exact cause of fever dreams. But some scientists think fevers cause your brain to “overheat,” affecting your cognitive processing, which results in dreams stranger than your typical dreams. Fevers may also interrupt your REM sleep cycle, which can lead to unusual dreams.
A note from Cleveland Clinic
While a fever can be uncomfortable, it’s typically a sign that your body is working properly and your immune system is kicking into action. You don’t need to treat low fevers, but you can take over-the-counter pain relievers until they pass. If you or your child has a higher temperature or your fever hasn’t passed within a few days, you should make a call to your healthcare provider. While infections most often cause fevers, there could be other underlying conditions you’ll want to get checked out.
Last reviewed on 05/31/2023.
Learn more about the Health Library and our editorial process.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy
