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Fever: What’s Causing It?

Fever may spike for a variety of reasons—some not serious, others more so.
Fever — A blue thermometer with yellow fluid and two light blue measurement lines on the upper left of the thermometer.
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Written by Petrina Craine, MD.
Assistant Clinical Professor of Emergency Medicine, Columbia University
Medically reviewed by
Last updated April 25, 2024

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Fever quiz

Take a quiz to find out what's causing your fever.

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What's a fever?

When your body temperature rises above normal, you have a fever. Normal varies among us, but is about 97°F to 99°F. But body temperature can fluctuate throughout the day, when exercising, during menstrual cycles, and for other reasons unrelated to being sick.

A fever occurs when the hypothalamus (a sensor area of our brain) increases our body temperature, usually to help “burn off” a viral or bacterial infection. A fever is often accompanied by other symptoms like feeling flushed, sweating, getting chills, muscle aches, and fatigue.

Illnesses that can cause a fever range from the common cold and sore throat to more serious conditions like pneumonia and appendicitis. A persistent low-grade fever may be the first sign of diseases such as lupus or certain cancers like lymphoma.

Fever is different from hyperthermia. Hyperthermia is when the body can no longer regulate its temperature. Temperature can rise to over 104°F. It's like your body’s internal thermostat has stopped working and the environmental temperature has taken over. Hyperthermia is a life-threatening emergency—go to the emergency room immediately for further care.

Fever can usually be treated with over-the-counter fever reducers like acetaminophen (Tylenol) or ibuprofen (Advil or Motrin).


1. Mononucleosis


Mononucleosis, or mono, is a highly contagious viral infection typically caused by the Epstein-Barr virus. Though it can be caused by other viruses (such as cytomegalovirus). Mono spreads through saliva, which is why it’s often called “the kissing disease.” But it can also be spread by sneezing, coughing, and sharing things like water bottles.

It’s a viral infection, so antibiotics won’t treat it. Symptoms can last as long as 1 to 2 months. Try to get plenty of rest (you’ll be tired) and take it easy. Drink lots of fluids and take ibuprofen (Advil) or acetaminophen (Tylenol) to bring down your fever.

2. Influenza (flu)


  • Fever
  • Cough
  • Congestion
  • Headache
  • Body aches
  • Nausea, vomiting, diarrhea (especially in young children)

The flu is an infection of the respiratory tract (e.g.,your lungs, nose, and mouth) caused by the influenza virus. There are different types of flu viruses, which is why it’s important to get a flu shot every year.

Most people who get the flu recover in a few days. But some develop complications like pneumonia. This can be life-threatening, especially for the elderly or people with compromised immune systems.

If you are at risk of complications, call your doctor right away or go to urgent care. You may be prescribed an antiviral medication, which can reduce the number of days you are sick and your risk of complications.

Otherwise, treat your symptoms as needed. You can take over-the-counter pain relievers like acetaminophen or ibuprofen. Get plenty of rest and drink lots of fluids.

It’s important to see a doctor if symptoms don’t get better, you can’t drink enough fluids, or your fever lasts for longer than 3 to 5 days.

Dr. Rx

Medicines are often mixed with other medicines, such as over-the-counter medicines for the common cold and for the flu. Unintentional overdoses can and do happen from mixing multiple combinations of these medicines. It is best to choose one fever-reducing medicine to take during your illness. —Dr. Petrina Craine

3. Pneumonia


  • Fever
  • Cough (which may also produce mucus/phlegm)
  • Trouble breathing or increased breathing rate
  • Fatigue
  • Chest pain
  • Nausea, vomiting, loss of appetite, and diarrhea (especially in younger children)
  • Confusion (especially in the elderly)

Pneumonia is an infection in your lungs. It’s most commonly caused by viruses and bacteria. The infection irritates the air sacs. They fill with pus and other fluids, making it more difficult to breathe. Colds, bronchitis, influenza, and other respiratory infections can lead to pneumonia if bacteria starts to breed in your lungs.

Pneumonia can be life-threatening, especially to the elderly or people with compromised immune systems, which is why immediate diagnosis and treatment is essential.

Pneumonia is often diagnosed with a chest x-ray. Pneumonia caused by bacteria can be treated with antibiotics.

You may have to be admitted to the hospital if you have low oxygen levels, severe dehydration, or other complications.

4. Common cold


A common cold is a viral infection of the respiratory tract (usually just your nose and mouth). Many types of viruses can give you a cold, but rhinoviruses are a common culprit.

Most people have relatively mild symptoms and recover in a few days. Over-the-counter medications, such as decongestants (such as pseudoepinephrine) and natural foods like honey (in ages 2 and older), may relieve symptoms like congestion, cough, and sore throat. Resting and drinking a lot of fluids can also help you feel better.

5. Sinusitis


  • Fever
  • Runny nose
  • Congestion
  • Facial pain or pressure
  • Headache
  • Cough that may produce mucus
  • Foul-smelling breath or taste in mouth

The sinuses are small air pockets in your forehead, nose, cheekbones, and in between your eyes. Normally they produce thin mucus to help trap foreign materials. When infected with viruses or bacteria, sinuses produce more fluid and it can get thicker—causing blockages. That can lead to infection, called sinusitis. Most sinus infections are caused by viruses, so antibiotics won’t help treat symptoms. If your symptoms last longer than 10 days or get worse after 5 to 7 days, you may have bacterial sinusitis and will need to see a doctor who may prescribe antibiotics. 

Over-the-counter decongestants (such as pseudoephedrine) should be used with caution. Taking them for more than 3 days increases the risk of the congestion coming back. Sometimes even worse than before. Monitor symptoms and make sure to drink lots of fluids to help relieve pressure.

6. Strep throat


Strep throat is a throat infection caused by a bacteria called group A Streptococcus. Strep throat is contagious and more common in children than adults. A hallmark symptom is a very sore throat that may make eating, talking, or swallowing difficult. Sometimes strep throat is associated with a rash that looks like sandpaper—it’s known as scarlet fever.

Strep throat is diagnosed with a test and is treated with antibiotics. If left untreated, it can turn into a more serious illness such as rheumatic fever. Antibiotics may make you feel better in the next few days but it’s important to take the antibiotics for the full course prescribed.

7. Gastroenteritis (stomach flu)


Gastroenteritis, or stomach flu, is inflammation of your gastrointestinal (GI) organs, like your stomach and intestines. It is usually caused by infection with a contagious virus, such as the rotavirus or norovirus. Most people recover in a few days at home.

It is very important to drink fluids as the body can lose a lot of water and electrolytes from throwing up or diarrhea. If you think you’re dehydrated or cannot keep any foods or liquids down, talk to your doctor. Your doctor may recommend hospitalization for monitoring and IV fluids for rehydration.

Also, gastroenteritis can sometimes mimic other more serious conditions such as appendicitis, which often has symptoms like fever, nausea and vomiting, and belly pain. Call your doctor when in doubt about the cause of your symptoms.

8. COVID-19


  • Fever
  • Loss of taste or smell
  • Cough or congestion, which may produce mucus
  • Runny nose
  • Nausea, vomiting, or diarrhea
  • Chest pain
  • Fatigue
  • Body aches
  • Chills
  • Headache
  • Low oxygen levels on a pulse oximeter (oxygen measuring device)

COVID-19 is a new, highly contagious virus caused by a coronavirus, specifically the SARS-CoV-19. This virus infects the respiratory tract (including your nose, throat, sinuses, and lungs). But it can also affect other organs including the brain, blood vessels, and the skin.

Symptoms are very similar to those of the flu. Sometimes, COVID-19 begins with mild symptoms that become life-threatening.

Rest and drinking lots of fluids can be important in treating symptoms and feeling better for people with mild cases of COVID-19. If you suspect COVID-19, monitor the progression of your symptoms and keep your doctor updated about your symptoms.

Your doctor will recommend additional at-home treatment plans, which may include over-the-counter medication, oxygen monitoring devices, or prescription medication, like an inhaler. They can also advise you when to go to the hospital if your symptoms worsen.

9. Cancer


  • Fever that comes and goes
  • Unexplained weight loss
  • Easily bruised skin
  • Swollen lymph nodes
  • Chills
  • Fatigue

Cancer can cause a persistent low-grade fever, as well as chills. This is especially true for certain blood cancers like leukemia, lymphoma, or myeloma. See a doctor if a fever doesn’t break, if you have a continual low-grade fever or a low-grade fever that seems to come and go.

Cancer may cause other symptoms, such as unexplained weight loss, swollen lymph glands, or fatigue regardless of how much sleep you’ve been getting. Blood cancers may also cause easily bruised skin or bleeding gums.

Treatment depends on the type of cancer and how far it has progressed. Treatment may include chemotherapy, radiation, and/or surgery.

10. Autoimmune disease


  • Fatigue
  • Fever (usually low-grade)
  • Swollen glands
  • Joint pain
  • Digestive issues
  • Skin or hair changes

Various autoimmune diseases such as lupus and rheumatoid arthritis may cause a low-grade fever. Autoimmune disorders can interfere with daily life and can make it hard to manage daily tasks without pain. They can also make you feel exhausted, depressed, and lower your quality of life.

Autoimmune disorders can have a range of symptoms and can be complicated to diagnose. Often, diagnosis is made by ruling out other causes. Treatment can include symptom management, medication, and immunotherapy.

Other possible causes

A number of conditions may also cause fever. These include:

  • Diseases transmitted by ticks like Lyme disease
  • Appendicitis
  • Urinary tract infection (fever is usually a sign that the upper urinary tract—like the kidneys—is affected)
  • Extreme sunburn (rare)
  • Food poisoning (usually a mild fever occurs if any)
  • Blood clots (rare)
  • Tuberculosis

Pro Tip

Knowing the measured number on the thermometer and how the temperature was taken (e.g. from the mouth or from the forehead) are both incredibly helpful tidbits to tell your doctor when caring for someone with a fever. It’s also important to let your doctor know if any medicines were given to help reduce fever and what time they were taken. —Dr. Craine

When to call the doctor or visit the ER

  • You have a moderate to severe fever—usually 102°F and greater.
  • You have a fever that has been persistent (greater than 3 to 5 days).
  • You have a weakened immune system (elderly adults, people with cancer, people on medications that suppress the immune system, people with chronic conditions like diabetes, etc.). Even an elevated temperature above 100°F could be the first sign of illness.
  • You recently had surgery or some other procedure.
  • You are pregnant.
  • You think you may have been exposed to a tick.

The list above may also mean a trip to the ER is necessary. You should also visit the ER for the following:

  • You have extreme pain (including neck, head, and back pain).
  • You’re feeling confused or having hallucinations.
  • You’re vomiting blood or you have bloody stool.
  • You have trouble breathing.
  • You’re experiencing severe hives or swelling.
  • You have a seizure.
  • You are having trouble eating and drinking enough to stay hydrated.

When to call the pediatrician

It's important to get an accurate temperature reading in children, especially infants. Ear and forehead thermometers provide less accurate readings than oral and rectal thermometers.

Call the pediatrician in the following circumstances:

  • Fever doesn’t break even with fever-reducing medications.
  • Child or toddler isn’t eating or drinking.
  • Child or toddler is inconsolable and seems to be in pain.
  • You have an infant between 3 and 6 months with a fever greater than 102°F.

If your infant is younger than 3 months, the American Academy of Pediatrics advises calling your pediatrician or going to an ER immediately if your infant has a fever greater than 100.4°F, even if they have no other symptoms. Infants who are less than 1 month old or have other risk factors for serious illness should see a doctor for any fever.

Pro Tip

Sometimes patients are especially fearful of a fever, especially a high one. They feel that the fever will cause a seizure in anyone. That’s fortunately not true. Febrile seizures—as they are called—are more likely to occur in children than adults. But only about 2%–5% of children with fever will ever experience a seizure from it. —Dr. Craine


At-home care

  • Bathe or shower with lukewarm water. The “old wives’ tale” about only using cold water is not necessary and may make you feel even worse.
  • Keep the room temperature at a comfortable level.
  • Drink plenty of fluids.
  • Take fever-reducing medicines like ibuprofen or acetaminophen.

Other treatment options

  • Medical testing to help identify the cause of fever.
  • IV hydration.
  • Hospital monitoring.
  • Prescription medication or additional treatments to treat the underlying cause of fever.

Fever can make anyone feel awful, but its health effects are often temporary and short-lived. Setting self-care goals such as staying hydrated and getting adequate rest are often all that most people need to do to help support their bodies during this time. Thankfully, most people with fever don’t experience long-term health complications from it.

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