Got a Fever? Possible Causes & When to See a Doctor

Understand your fever symptoms with Buoy, including 10 causes and common questions concerning your fever.

Fever symptoms

Headache? Chills? Fatigue? Uh-oh, you might just have a fever. A fever, even if it doesn't necessitate an immediate trip to the doctor (for most adults), does indicate something is definitely wrong in the body — from the flu to a cold, to something more serious [1,2].

This is because the body works very hard at maintaining homeostasis in all it processes, including body temperature, and when it cannot achieve that 98.6 degrees Fahrenheit like normal, there is always a reason. It's just that some reasons are more serious than others.

Common accompanying symptoms of fever

Fever symptoms are typically caused by other factors relating to the illness behind the fever and include:

  • Lack of appetite
  • Chills
  • Lethargy
  • Sensitivity to pain
  • Depression
  • Problems concentrating
  • Sleepiness
  • Sweating
  • Feeling hot then cold

Common accompanying symptoms of severe fever

With severe fever other symptoms may be present:

  • Delirium
  • Chatter and nonsensical babbling
  • Irritability
  • Fighting someone trying to help you
  • Confusion
  • Passing out
  • Seizures

The hallmark sign of a fever is a temperature at or above 100.4 degrees Fahrenheit (38 degrees Celsius). Normal body temperature ranges from 97 to 99 degrees Fahrenheit (which is the same as 36.1 to 37.2 degrees Celsius).

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Fever causes and conditions

Our body temperature is controlled in a way similar to the way that a thermostat regulates the temperature of a room. When a room cools below a set temperature, a sensor in the thermostat sends an electronic signal to the furnace to produce more heat. In much the same way, when our body temperature drops below normal, "sensors" in the hypothalamus (a gland in the brain) are triggered to increase body temperature. One way the body increases temperature is by shivering.

Those same "sensors" can indicate that the body is too warm. Whether due to environmental conditions, such as working outside on a hot day, or pathologic conditions, such as fever, mechanisms (such as sweating) are put into action to lower body temperature.

Infectious causes

Fever symptoms usually mean infection commonly the flu, the common cold, or pneumonia. In fact, any significant infection will cause a fever. The infection, whether it be bacterial, viral, or from a parasite, causes the thermostat in the hypothalamus to reset. As body temperature rises, your immune system is stimulated to clear the infection.

In fact, an elevated body temperature (fever) is how our body protects us via the immune system as it combats our infection and "burns" it out of the body. Usually, this rise in body temperature helps us beat the infection.

Other causes

Trauma, surgery, some cancers, certain types of nervous system injuries and some medications may also cause fever symptoms.

This list does not constitute medical advice and may not accurately represent what you have.

Common cold

The common cold is a viral infection of the upper respiratory tract, which includes the nose, mouth, sinuses, throat, and larynx. There are over 200 viruses that can cause upper respiratory infections, and usually the exact virus behind a cold is never known.

The common cold is, of course, very common..

Viral (norovirus) infection

If you ever heard of an entire cruise ship of people coming down with the same “stomach bug,” chances are that was norovirus. Fortunately, norovirus usually goes away on its own after a few days, but is pretty unpleasant and can spread extremely easily. The ..


Influenza, or "flu," is a contagious respiratory illness caused by viruses. It is spread through the air by coughing, sneezing, or even talking.

Anyone can get the flu, but those who are very young, over 65, and/or have pre-existing medical conditions are most at risk for complications.

Symptoms include fever, chills, cough, sore throat, body aches, and extreme fatigue. The symptoms may appear very suddenly.

Flu can bring on secondary bacterial infections in the lungs or ears. Dehydration is a great concern because the patient rarely wants to eat or drink.

Diagnosis is usually made by symptoms. There are tests that use a swab taken from the nose or throat, but they are not always accurate or necessary.

Treatment consists mainly of good supportive care, which means providing the patient with rest, fluids, and pain-relieving medication such as ibuprofen. Do not give aspirin to children.

Antibiotics cannot help with the flu, since antibiotics only work against bacteria. There are anti-viral medications that a doctor may prescribe.

The best prevention is an annual flu shot.

Rarity: Common

Top Symptoms: fatigue, headache, loss of appetite, cough, muscle aches

Symptoms that never occur with influenza: headache resulting from a head injury

Urgency: Phone call or in-person visit

Viral throat infection

A viral throat infection is an infection of the throat, or pharynx, that is caused by viruses. Viruses are different from bacteria such as Streptococcus pyogenes (which causes "strep throat"). Viral infections are the most common cause of sore throats in children and adu..

Viral pneumonia

Viral pneumonia, also called "viral walking pneumonia," is an infection of the lung tissue with influenza ("flu") or other viruses.

These viruses spread through the air when an infected person coughs or sneezes.

Those with weakened immune systems are most susceptible, such as young children, the elderly, and anyone receiving chemotherapy or organ transplant medications.

Symptoms may be mild at first. Most common are cough showing mucus or blood; high fever with shaking chills; shortness of breath; headache; fatigue; and sharp chest pain on deep breathing or coughing.

Medical care is needed right away. If not treated, viral pneumonia can lead to respiratory and organ failure.

Diagnosis is made through chest x-ray. A blood draw or nasal swab may be done for further testing.

Antibiotics do not work against viruses and will not help viral pneumonia. Treatment involves antiviral drugs, corticosteroids, oxygen, pain/fever reducers such as ibuprofen, and fluids. IV (intravenous) fluids may be needed to prevent dehydration.

Prevention consists of flu shots as well as frequent and thorough handwashing.

Rarity: Uncommon

Top Symptoms: fatigue, headache, cough, shortness of breath, loss of appetite

Urgency: Primary care doctor

Mononucleosis infection

Infectious mononucleosis, also called "mono" or "kissing disease," can be debilitating for a while but is usually not dangerous in itself.

Several viruses cause mononucleosis. It spreads easily through saliva and other body fluids. Sharing a drinking glass or a spoon, or kissing someone who has the virus – even they show no symptoms – will transmit the disease. It can also be sexually transmitted.

Due to lifestyle, teenagers and young adults seem to be the most susceptible.

Symptoms include tiredness, sore throat, fever, rash, body aches, swelling in the neck and armpits, and sometimes swollen liver and spleen. The symptoms alone are usually enough for the doctor to make a diagnosis.

Because mononucleosis is caused by a virus, antibiotics will not help. Treatment consists of bed rest, fluids, and good nutrition. The patient should still be under a doctor's care due to the risk of secondary infections or damage to the heart, liver, and spleen.

Handwashing, cleanliness, not sharing dishes or drinking glasses, and not having unprotected sex are the best ways to prevent mononucleosis.

Rarity: Common

Top Symptoms: fatigue, headache, loss of appetite, abdominal pain (stomach ache), cough

Urgency: Primary care doctor

Bacterial pneumonia

Bacterial pneumonia is an infection of the lungs caused by one of several different bacteria, often Streptococcus pneumoniae. Pneumonia is often contracted in hospitals or nursing homes.

Symptoms include fatigue, fever, chills, painful and difficult breathing, and cough that brings up mucus. Elderly patients may have low body temperature and confusion.

Pneumonia can be a medical emergency for very young children or those over age 65, as well as anyone with a weakened immune system or a chronic heart or lung condition. Emergency room is only needed for severe cases or for those with immune deficiency.

Diagnosis is made through blood tests and chest x-ray.

With bacterial pneumonia, the treatment is antibiotics. Be sure to finish all the medication, even if you start to feel better. Hospitalization may be necessary for higher-risk cases.

Some types of bacterial pneumonia can be prevented through vaccination. Flu shots help, too, by preventing another illness from taking hold. Keep the immune system healthy through good diet and sleep habits, not smoking, and frequent handwashing.

Rarity: Common

Top Symptoms: fatigue, cough, headache, loss of appetite, shortness of breath

Symptoms that always occur with bacterial pneumonia: cough

Urgency: In-person visit

Strep throat requiring throat swab

Strep throat, or "strep," is a sore throat specifically caused by the bacteria Streptococcus pyogenes, also called group A streptococcus.

The illness spreads when an infected person coughs or sneezes and then someone else inhales the airborne bacteria, or touches a surface where it has landed and then touches their own face.

Children are most susceptible but anyone can be infected.

Symptoms include sudden throat pain, fever, headache, rash, body aches, and red, swollen tonsils. These symptoms can be caused by other illnesses, so a sample is taken by gently rubbing a sterile cotton-tipped swab over the back of the throat.

Testing will identify the organism responsible so that treatment with the appropriate antibiotic can begin. Be sure to finish all of the medication as directed, even after feeling better.

Untreated strep throat can lead to ear infections, kidney disease, scarlet fever, and rheumatic fever. These are serious illnesses. If strep throat is suspected, the person should see a doctor as soon as possible.

Rarity: Common

Top Symptoms: fatigue, nausea, sore throat, fever, rash

Symptoms that always occur with strep throat requiring throat swab: sore throat

Symptoms that never occur with strep throat requiring throat swab: general weakness

Urgency: Primary care doctor

Acute bacterial sinusitis

Acute bacterial sinusitis, also called bacterial rhinosinusitis or "sinus infection," has symptoms much like viral rhinosinusitis but a different treatment.

Any sinusitis usually begins with common cold viruses. Sometimes a secondary bacterial infection takes hold. Like cold viruses, these bacteria can be inhaled after an infected person coughs or sneezes.

Anyone with viral sinusitis, upper-respiratory allergy, nasal passage abnormality, lung illness, or a weakened immune system is more prone to bacterial sinusitis.

Symptoms include thick yellowish or greenish nasal discharge; one-sided pain in the upper jaw or teeth; one-sided sinus pain and pressure; fatigue; fever; and symptoms that get worse after first improving.

See a doctor right away for severe headache, high fever, stiff neck, or vision changes. These can indicate a medical emergency.

Diagnosis is made with a simple examination in the doctor's office.

Bacterial sinusitis can be treated with antibiotics, but this is not always necessary. Often rest, fluids, and over-the-counter pain relievers and decongestants are enough.

Prevention is done through good lifestyle and hygiene to keep the immune system strong.

Rarity: Common

Top Symptoms: fatigue, headache, cough, sinusitis symptoms, muscle aches

Symptoms that always occur with acute bacterial sinusitis: sinusitis symptoms

Symptoms that never occur with acute bacterial sinusitis: clear runny nose, being severely ill

Urgency: Primary care doctor

Urinary tract infection

A urinary tract infection is an infection of the urinary tract, which includes the kidneys, bladder, and urethra. Urinary tract infections are usually caused by infections by fecal bacteria.

Symptoms of urinary tract infections include pain with urination (dysuria), ..

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Fever treatments and relief

When to see a doctor for a fever

Doctors typically agree that mild fevers require no effort to bring the fever down or necessitate a trip to a hospital. In fact, leaving the fever alone helps the body continue to fight the infection causing the fever. The fever is helping the body to kill the viruses and infectious bacteria that is causing the fever.

How to take a temperature

For taking your temperature, know that temperature can be measured with several types of thermometers. There are oral, rectal, ear, forehead, and underarm thermometers. Although oral and rectal temperatures are the most accurate, ear, forehead and underarm temperatures are the easiest to take (especially with infants/children). It is normal for an oral or rectal temperature to be a bit higher than one taken from the ear, forehead or underarm. It is important to wait at least 30 minutes after eating or drinking anything before taking an oral temperature.

Medications to lower a fever

The following medications can help to lower a fever if necessary:

  • Aspirin: This should not be given to children or pregnant women.
  • Acetaminophen (Tylenol)
  • Aspirin and acetaminophen taken together are more effective than either taken alone
  • Ibuprofen (Advil, Motrin)
  • Naproxen (Aleve)
  • Corticosteroids: Use as prescribed by a healthcare provider.

Other at-home treatments for fever

Overall, getting adequate rest and drinking plenty of fluids is the best approach to most fevers.

When fevers are an emergency

Seek professional care immediately if you have fever and:

  • Are pregnant
  • Are having hallucinations or seizures
  • Your temperature does not go down: Even after taking a fever-reducing medication
  • Have recently traveled abroad: Especially to resource-poor areas
  • Have recently had surgery
  • Have a fever that lasts for more than three days
  • Have a chronic medical condition: Such as diabetes, cancer or sickle cell disease
  • Have a weakened immune system
  • Think you may have recently been bitten by a tick
  • Your temperature is more than 104 degrees Fahrenheit
  • Are experiencing the following: Confusion, rash, persistent vomiting or diarrhea, problems breathing, severe neck or head pain, or severe abdominal pain

FAQs about fever

What is considered a fever?

In adults, a fever is a temperature higher that 38.2 degrees Celsius or 100.4 degrees Fahrenheit. A fever is caused by "pyrogens," pyro meaning heat or fire, and gen, meaning the creation of substances that create heat. Pyrogens are often foreign substances that the body recognizes as dangerous. In response, the body raises the temperature to impair the functioning of those compounds throughout the body.

What is a bad (high-grade) fever?

A high grade fever is commonly described at 39.4 degrees Celsius or 103 degrees Fahrenheit. At more than 104 degrees Fahrenheit, it is often necessary to seek medical attention immediately. A fever is caused by the hypothalamus (the thermostat of the body) setting a new base temperature. The body, because the body now believes its temperature should be five, 10, or 15 degrees higher than normal, feels cold relatively speaking. This causes chills, the impulse to bundle up, shivering, and (as blood vessels constrict to keep warm blood near the chest and abdomen) cold hands and feet.

What causes low grade fever?

Fevers, low and high grade fevers, are caused most commonly by infection. The body is able to tolerate higher temperatures than the microbes infecting it. The body elevates its temperature to a range wherein microbes are unable to function, but the body is still functional. When the microbes are no longer functioning, the fever resolves.

Why do we get fevers?(same as previous)

Fevers are our body's defense mechanism. They are a means of raising the body temperature to an extent that pathogens (bacteria and viruses) and the cells they invade do not function as well as they have in the past. We get fevers primarily ro raise our body temperature which may interfere with the function of pathogens within the body and give our immune cells a competitive edge.

How long is too long to have a fever?

Most common colds and illnesses last for 710 days. In the presence of symptoms like a cough, sore throat, or runny nose, a fever should last no longer than the symptoms, and usually lasts for 34 days during each cold. If an individual has a fever in the absence of symptoms for longer than three weeks, it is called a fever of unknown origin and that individual should seek care.

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Take a thorough self-assessment on what you may have

Questions your doctor may ask about fever

  • Have you been feeling more tired than usual, lethargic or fatigued despite sleeping a normal amount?
  • Have you experienced any nausea?
  • Do you have a cough?
  • Are you experiencing a headache?

Self-diagnose with our free Buoy Assistant if you answer yes on any of these questions.

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  2. Informed Health Online [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006. Fever in Children: Overview. Updated Nov 17, 2016. NCBI Link.
  3. Informed Health Online [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006. How is Body Temperature Regulated and What is Fever? Updated Nov 17, 2016. NCBI Link.
  4. Ray JJ, Schulman CI. Fever: Suppress or Let It Ride? Journal of Thoracic Disease? 2015;7(12):E633-E636. NCBI Link.
  5. Consolini DM. Fever in Infants and Children. Merck Manual Consumer Version. Updated July 2018. Merck Manual Consumer Version Link.