A fever is a body temperature that’s higher than normal. It’s one of the most common reasons people of all ages seek medical advice, and for good reason — it’s the body’s way of signaling that something needs attention.
Fever is usually a response to an infection, but it isn’t a disease on its own, and it isn’t limited to infections. According to MedlinePlus, a service of the National Library of Medicine at the NIH, a fever is a sign that your body is trying to fight an illness, and infections cause most fevers, though other causes exist too.
How concerning a fever is depends on more than the number on the thermometer. Age, how high the temperature is, how long it lasts, and what other symptoms show up all matter. This guide covers what causes fever, how to measure it accurately, what you can do at home, and — importantly — when it’s time to get medical help.
What Is a Fever?
Your body has a built-in thermostat, located in a part of the brain called the hypothalamus, that keeps your temperature within a narrow, stable range.
How temperature regulation works
When your immune system detects an infection or another trigger, it can raise your hypothalamus’s temperature “set point.” Your body then works to reach that higher setpoint, sometimes causing chills as you warm up. This regulated rise is what doctors call a fever.
Feeling hot versus having a fever
Feeling warm, flushed, or sweaty doesn’t always mean you have a fever. The only reliable way to know is to measure your temperature with a thermometer, since how you feel can be affected by activity, clothing, room temperature, and emotions.
Why body temperature naturally varies
Normal body temperature isn’t a single fixed number. According to MedlinePlus, the average normal body temperature is 98.6°F (37°C), but it can vary by as much as 1° to 2°F throughout the day in otherwise healthy people, and it’s typically lowest in the morning and highest in the evening.
What Temperature Is Considered a Fever?
Fever thresholds vary depending on how and where the temperature is measured, along with age and clinical context. According to Mayo Clinic, these readings are generally considered a fever:
- Oral (mouth) temperature of 100.4°F (38°C) or higher
- Rectal, ear, or temporal artery temperature of 100.4°F (38°C) or higher
- Armpit temperature of 99°F (37.2°C) or higher
There’s no single number that applies to every person and every situation, so the method used and the person’s age and health both factor into how a given reading should be interpreted.
What Are the Symptoms of a Fever?
Fever itself is a symptom, but it often comes with other signs as your body responds to whatever is triggering it. Common symptoms include:
- Feeling hot or cold
- Chills
- Sweating
- Headache
- Muscle or body aches
- Fatigue
- Weakness
- Reduced appetite
- Increased heart rate
- Feeling generally unwell
Which of these symptoms show up, and how severe they are, depends heavily on the underlying cause. Our guide to what causes headaches and our overview of what causes fatigue cover these individual symptoms in more depth if either is your main concern.
Fever and Chills
It can seem strange to feel cold while your temperature is actually rising, but this happens because your body is trying to reach a new, higher set point. Shivering and chills are your muscles working to generate heat and catch up to that new target.
Fever and Sweating
Once your temperature reaches the new set point, or as the underlying trigger resolves and the set point drops back down, your body sweats to cool itself back toward normal. This is why fevers often “break” with noticeable sweating.
What Causes Fever?
Fever has many possible causes, ranging from common infections to less common noninfectious triggers.
Viral Infections
Viral infections are among the most frequent causes of fever. Common respiratory viruses, influenza, COVID-19, and other viral illnesses can all trigger a fever, often alongside cough, congestion, sore throat, fatigue, or body aches. If a cough is part of your symptoms, our guide to what causes a cough covers this symptom in more detail.
Bacterial Infections
Bacterial infections can also cause fever, including some throat infections, urinary tract infections, pneumonia, and certain skin infections. Fever alone can’t tell you or a doctor whether an infection is bacterial or viral — that distinction typically requires an exam and sometimes testing.
Other Infections
Depending on your location and travel history, other infections can also cause fever, including gastrointestinal infections and certain mosquito- or tick-borne illnesses. If you’ve traveled recently or live in an area where these infections are more common, mentioning that history to a doctor helps guide appropriate testing.
Vaccinations
A mild, temporary fever is a common and expected response after some vaccines, reflecting the immune system building protection. This type of fever is usually short-lived and resolves on its own within a day or two.
Heat-Related Illness
Fever is different from a heat-related illness like heat exhaustion or heat stroke, even though both involve a high body temperature. Heat stroke happens when the body’s cooling mechanisms fail entirely under external heat stress, and according to the CDC, it’s a medical emergency requiring an immediate call to 911, not home management like an ordinary fever.
Medications and Other Causes
Some medications can cause fever as a side effect, sometimes called drug fever. According to MedlinePlus, other noninfectious causes of fever include certain autoimmune diseases and, less commonly, some cancers. A doctor can help sort out whether a medication or another noninfectious cause might be responsible if infection has been ruled out.
Fever in Babies, Children, Adults & Older Adults
How much a fever matters, and how quickly it needs attention, depends heavily on the age of the person who has it.
Fever in Babies and Children
Fever in very young infants deserves prompt medical attention. The American Academy of Pediatrics’ clinical practice guideline specifically addresses evaluation of well-appearing infants between 8 and 60 days old with a fever at or above 100.4°F (38°C), reflecting that young infants can have a serious infection without other obvious signs. Any fever in an infant under 3 months old should be discussed with a pediatrician promptly, since guidance for this age group differs from older children.
For older infants and children, how the child looks and acts matters alongside the number on the thermometer. Pay attention to behavior and alertness, hydration (wet diapers, tears when crying, dry mouth), breathing, associated symptoms like rash or ear pain, and how long the illness has lasted. A child who is drinking fluids, alert, and responsive despite a fever is generally reassuring, even with a fairly high number, while a child who seems very sick or is difficult to wake deserves prompt evaluation regardless of the exact temperature.
Fever in Adults and Older Adults
In adults, fever is most often caused by a common infection and resolves within a few days. Warning signs that warrant closer attention include a very high temperature, fever lasting more than a few days, or fever accompanied by severe symptoms, which the sections below cover in detail.
Fever in older adults deserves extra attention for a specific reason: serious infections don’t always cause the same degree of fever in this age group, and some older adults mount a smaller temperature response even during a significant infection. Underlying health conditions, medications, and a weaker immune response can all affect how fever presents, so a normal-seeming temperature shouldn’t rule out a serious problem in an older adult who seems unwell.
When Should You Worry About a Fever?
Concern about a fever depends on more than the temperature alone. Several factors can raise the level of concern:
- A very high temperature
- Fever that persists or keeps getting worse
- Fever that repeatedly comes and goes
- Severe accompanying symptoms
- Signs of dehydration
- Significant underlying medical conditions
- Very young age
- Older age or frailty
- Recent surgery
- A weakened immune system
- Recent travel or relevant exposure to infection
Not every fever needs treatment or an emergency visit. Most fevers in otherwise healthy people resolve on their own with rest and supportive care. The factors above are meant to help you recognize when a fever is more likely to need a closer look, not to suggest that every fever is cause for alarm.
When Is a Fever a Medical Emergency?
Most fevers aren’t emergencies, but some accompanying symptoms mean you should seek urgent or emergency medical care right away. Contact your local emergency service or go to an emergency room if fever occurs with:
- Difficulty breathing
- Severe chest pain
- New confusion or difficulty waking
- A seizure
- Fainting
- Severe dehydration
- Severe headache with other neurological symptoms
- A stiff neck combined with feeling very unwell
- A rapidly spreading or concerning rash
- Severe abdominal pain
- Persistent vomiting with an inability to keep fluids down
- Signs of a serious allergic reaction
- A fever in a very young infant, per pediatric guidance discussed above
If chest pain or shortness of breath accompanies a fever, these combinations deserve prompt evaluation. Don’t wait to see whether symptoms improve on their own when any of these warning signs are present.
When Should You See a Doctor for a Fever?
Beyond the emergency warning signs above, a range of situations call for a doctor’s evaluation, even if they aren’t immediately life-threatening. Contact a healthcare provider if you notice:
- Fever that persists longer than expected for a typical illness
- Fever that repeatedly returns after seeming to resolve
- Symptoms that are getting worse instead of better
- Significant dehydration
- Persistent vomiting or diarrhea
- Severe pain
- Breathing difficulties
- A new rash
- Concerning urinary symptoms, like pain or burning
- Fever after recent surgery
- Fever in someone with a weakened immune system
- Fever following relevant travel or a known infectious exposure
As a general guideline, adults with fever lasting more than a few days, or any fever in a child that isn’t improving after a couple of days, is a reasonable point to check in with a healthcare provider rather than continuing to wait it out.
How Do You Check for a Fever?
Getting an accurate temperature reading depends on using the right tool the right way.
Common measurement methods
According to MedlinePlus, digital thermometers are the most commonly recommended option today, and temperature can be measured several ways:
- Oral (in the mouth): A common method for older children and adults
- Rectal: Considered the most accurate for infants and young children
- Ear (tympanic): Convenient, though technique affects accuracy
- Temporal artery (forehead): Fast and non-invasive
- Armpit (axillary): The least invasive, but generally the least precise
Readings can differ depending on the method: rectal and ear temperatures tend to run about 0.5°F to 1°F higher than oral readings, while armpit readings tend to run about 0.5°F to 1°F lower.
Tips for Accurate Temperature Measurement
- Follow the thermometer manufacturer’s instructions closely.
- Use the same measurement method consistently when tracking a fever over time, since switching methods can make trends hard to interpret.
- Record the temperature and time when it’s useful to track a pattern.
- Weigh the person’s overall symptoms and behavior alongside the number, not instead of it.
How Is a Fever Treated?
Treatment for fever depends on the underlying cause and the person’s age and health. Not everyone with a fever needs medication.
Supportive care is often enough for a mild fever tied to a common viral illness, and may include:
- Rest
- Adequate fluids
- Comfortable, lightweight clothing and a comfortably cool room
- Treating the underlying infection or condition, when a specific treatment applies
- Fever-reducing medicine, when appropriate
Fever-Reducing Medicines
Over-the-counter fever reducers like acetaminophen and ibuprofen are commonly used to ease discomfort associated with fever. A few general safety points apply broadly:
- Follow the product label or your clinician’s instructions for dosing.
- Dosing often depends on age and weight, particularly for children.
- Some fever-reducing medicines aren’t suitable for certain people, including those with specific health conditions or who are taking other medications.
- Avoid combining products that contain the same active ingredient, since this can lead to accidentally taking too much.
- Children should receive age-appropriate medication and dosing guidance from a pediatrician or pharmacist rather than an adult dose scaled down informally.
If you’re unsure which medication is appropriate, or how much to give a child, a pharmacist or doctor can advise based on the specific product and the person’s age and weight.
What Can You Do at Home for a Fever?
For a mild fever without emergency warning signs, a few home measures can help you stay comfortable while your body fights off the underlying cause.
- Drink fluids regularly to stay hydrated.
- Rest as much as you’re able to.
- Wear comfortable, lightweight clothing.
- Keep your environment comfortably cool, not cold.
- Monitor your temperature periodically if it’s helpful for tracking your illness.
- Watch for any new or worsening symptoms.
- Continue any prescribed treatment if you already have a diagnosed underlying condition.
Getting enough rest supports your immune system’s ability to recover; our guide on how to improve your sleep has practical tips if illness is disrupting your usual sleep.
What Not to Do for a Fever
A few common habits can do more harm than good:
- Excessive bundling in blankets can trap heat and make you feel worse, rather than helping your body regulate its temperature.
- Extreme cold baths or ice water aren’t recommended for typical fever and can cause shivering, which may actually raise core temperature further.
- Taking antibiotics without a diagnosis doesn’t help viral fevers and contributes to antibiotic resistance; antibiotics only work against bacterial infections that a doctor has identified.
- Taking more fever-reducing medicine than directed doesn’t lower fever faster and increases the risk of side effects or overdose.
Is Fever the Same as Hyperthermia?
Fever and hyperthermia both involve a higher-than-normal body temperature, but they work differently.
Fever is a regulated increase in your body’s temperature set point, typically driven by your immune system’s response to infection or another trigger. Your body is deliberately raising its target temperature.
Hyperthermia is an abnormal rise in body temperature that happens without a corresponding change in the set point — your body is trying to stay at its normal temperature but external heat or another factor overwhelms its ability to do so. Heat stroke is a dangerous form of hyperthermia and is a medical emergency, unlike most ordinary fevers.
How Do Doctors Find the Cause of a Fever?
Identifying what’s behind a fever usually starts with a conversation, followed by an exam and, when needed, testing.
Medical History and Physical Exam
A doctor will typically ask about when the fever started, the highest temperature recorded, other symptoms, recent infections, travel, medication use, vaccination history, and any relevant environmental or occupational exposures. This history often narrows down the likely cause significantly before any testing begins. Depending on your symptoms, they’ll also assess your breathing, heart rate, blood pressure, hydration status, throat, lungs, abdomen, skin, and neurological status — each of which can offer clues about where an infection or other process may be located.
Diagnostic Tests
Depending on the suspected cause, a doctor may order blood tests, urine testing, respiratory testing, imaging, cultures, or other targeted tests. Testing is individualized based on your specific symptoms and medical history — not every fever requires extensive workup.
How Can You Prevent Some Causes of Fever?
While not every fever can be prevented, since your body’s fever response itself is protective, you can reduce your risk of many common infectious triggers.
- Practice regular hand hygiene, especially before eating and after coughing, sneezing, or using the bathroom. The CDC recommends washing with soap and water for at least 20 seconds.
- Stay up to date on recommended vaccinations.
- Follow food and water safety practices, especially while traveling.
- Avoid close contact with people who are ill when possible, and take precautions during travel, including researching destination-specific health risks.
- Use mosquito and tick protection in relevant areas and seasons.
- Manage chronic health conditions with your doctor’s guidance, since some conditions raise infection risk.
Supporting your overall health also plays a role in resisting infections generally. Our guide on how to live a healthy lifestyle and our collection of healthy living tips cover habits that support your immune system more broadly, and our overview of the most common health issues is a useful reference for understanding conditions that commonly affect Americans.
Frequently Asked Questions About Fever
What causes a fever?
Most fevers are caused by the body’s immune response to an infection, whether viral, bacterial, or another type. Other causes include some vaccines, certain medications, autoimmune conditions, and, less commonly, other underlying medical conditions.
What temperature is considered a fever?
Thresholds vary by measurement method: generally 100.4°F (38°C) or higher for oral, rectal, ear, or temporal artery readings, and 99°F (37.2°C) or higher under the arm, according to Mayo Clinic. The method used and the person’s individual baseline both matter when interpreting a reading.
How long does a fever usually last?
Most fevers tied to common viral infections last a few days to about a week. A fever that persists longer than expected, or that keeps returning, is worth discussing with a doctor.
Can you have a fever without an infection?
Yes. Vaccines, certain medications, autoimmune conditions, and some other medical conditions can all cause fever without an active infection being present.
Can stress cause a fever?
Severe emotional stress can sometimes cause a mild, temporary rise in body temperature in some people, though this is different from a fever caused by infection or inflammation. If you’re dealing with high stress levels generally, our guide on how to reduce stress covers practical techniques worth trying.
Should you always treat a fever?
No. Fever itself is often a helpful part of your body’s immune response, and mild fevers in otherwise healthy people frequently don’t require medication. Treatment decisions should weigh comfort, the underlying cause, and any individual risk factors.
Is a high fever dangerous?
A high number alone doesn’t automatically mean danger — how the person looks and feels, their age, and any accompanying symptoms matter just as much. That said, very high temperatures, or fever combined with any of the emergency warning signs covered above, should prompt urgent evaluation.
When should an adult see a doctor for a fever?
See a doctor if a fever lasts more than a few days, keeps returning, comes with severe symptoms, or occurs in someone with a significant underlying health condition or weakened immune system. Any of the emergency warning signs covered above warrant immediate care rather than a routine appointment.
When should a child with fever see a doctor?
Any fever in an infant under 3 months old warrants prompt medical evaluation, per current pediatric guidance. For older children, see a doctor if the fever persists, the child seems significantly unwell, shows signs of dehydration, or develops any of the emergency warning signs described above.
When is fever an emergency?
Fever is a medical emergency when it occurs with difficulty breathing, severe chest pain, confusion, a seizure, a stiff neck with severe illness, a concerning rash, severe abdominal pain, or persistent vomiting that prevents keeping fluids down, among other warning signs covered in this guide. Contact emergency services immediately if any of these occur.
What should you drink when you have a fever?
Water is generally the best choice, and options that also replace electrolytes, like oral rehydration solutions or diluted sports drinks, can help if sweating or reduced appetite has affected your fluid balance. Sipping fluids regularly, even in small amounts, is more effective than trying to drink a large volume at once if nausea is present.
Can a fever come and go?
Yes, some illnesses cause fever that rises and falls, sometimes called an intermittent fever. A fever that repeatedly returns after seeming to resolve is worth mentioning to a doctor, since this pattern can be relevant to identifying the underlying cause.
Sources
- MedlinePlus (National Library of Medicine, NIH). Fever
- MedlinePlus (National Library of Medicine, NIH). Fever — Medical Encyclopedia
- MedlinePlus (National Library of Medicine, NIH). Temperature Measurement
- Mayo Clinic. Fever: First Aid
- American Academy of Pediatrics. AAP Issues Clinical Practice Guideline for Fever in Infants Ages 8–60 Days
- Centers for Disease Control and Prevention. Warning Signs and Symptoms of Heat-Related Illness
- Centers for Disease Control and Prevention. When and How to Wash Your Hands
Medical Disclaimer
This article provides general health information and does not replace individualized medical evaluation or treatment. If you have concerns about a fever or any other symptom, talk to a qualified healthcare professional. If you experience any of the emergency warning signs listed above, seek immediate medical care.
Read our full Medical Disclaimer.