A fever is an elevation of body temperature. It is a symptom caused by a wide variety of illnesses.
A fever usually signals that an infection has invaded the body, but it can also be caused by heat exhaustion, autoimmune disorders, hormone imbalances, cancer, and some medications.


Additional symptoms associated with a fever vary and can aid in the determination of the underlying condition. Often, fevers can be relieved with over the counter treatments; however, a persistent or high grade fever will usually require evaluation by a medical provider. Identifying the signs and associated symptoms of a fever can help you seek the medical treatment appropriate for your condition.

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Normal Body Temperature

Human body temperature is a measure of the degree of heat present in the body. It represents the balance between the production of heat in the tissues and the loss of heat to the surroundings. Although body temperatures vary between individuals and depending on the environment, they will remain in a specific range known as the normal body temperature. This range is typically from 97°F (36.1°C) to 99°F (37.2°C), with an average body temperature of 98.6°F (37°C).

In order for the body to remain within the appropriate range, a temperature regulating system, or thermoregulatory mechanism, is vital. This system is controlled by the part of the brain known as the hypothalamus. The hypothalamus acts as a thermostat for the body as it works to maintain baseline temperatures in different environments. When external temperatures become too hot or too cold, nerve endings near the skin sense the change and communicate it to the brain. The hypothalamus then signals for changes in the body’s heat production and conservation in order to keep the temperature within it’s most comfortable range.

Measuring Body Temperature

Often, people who feel warm will automatically assume they have a fever, but this may not be the case. Using a digital thermometer to measure body temperature is the first step towards assessing a fever.

Digital thermometers can be used to measure body temperature in the mouth (oral), armpit (axillary), ear (tympanic membrane), or rectum (rectal). A temporal artery thermometer can also be used on the forehead to measure the temperature of the temporal artery.

Although oral temperatures and axillary temperatures are the most common form of measurement, rectal temperatures are the most accurate. A rectal temperature is closest to the body’s core temperature and is usually 1.8°F (0.6°C) higher than an oral temperature taken at the same time. Measuring a rectal temperature is most appropriate for children under the age of 5 years. However, this should be done with great caution to avoid tearing the rectum. If measuring your child’s rectal temperature, make sure to clean and coat the thermometer with petroleum jelly prior to use. Do not insert the thermometer more than an inch into the rectum and always grasp the thermometer firmly in place. If you do not feel comfortable taking your child’s rectal temperature, please consult a medical provider.

Temperatures can be measured in either Fahrenheit or Celsius. Familiarity with both standards of measurement is helpful, especially when discussing a fever with a medical provider.

Fahrenheit Celsius
96 35.5
97 36.1
98 36.6
99 37.2
100 37.7
101 38.3
102 38.9
103 39.4
104 40.0
105 40.5

What is a Fever?

A fever, also known as pyrexia, is an increase in temperature above the body’s comfortable range. It is not an illness itself, but rather a common symptom of a variety of different illnesses or conditions. Individuals with a fever are said to be febrile and individuals without a fever are afebrile.

For adults, a fever is deemed medically relevant at an oral temperature above 100.4°F (38°C). In children, an oral temperature above 99.5°F or a rectal temperature above 100.4°F (38°C) is considered medically significant. Temperatures ranging from 99°F to 101°F are termed low grade while temperatures from 101°F to 104°F are high grade.

Axillary/Temporal (°F) Oral (°F) Rectal (°F)
98.4–99.3 99.5–99.9 100.4–101
99.4–101.1 100–101.5 101.1–102.4
101.2–102 101.6–102.4 102.5–103.5
102.1–103.1 102.5–103.5 103.6–104.6
103.2–104 103.6–104.6 104.7–105.6

Treatment of Fever

The first line of defense against a fever and it’s associated discomfort is over the counter medications such as acetaminophen (Tylenol), ibuprofen (Advil or Motrin), and naproxen (Aleve). Aspirin is also an effective treatment for adults, but should not be given to children and adolescents under the age of 19, as it is linked to a potentially deadly condition known as Reye’s syndrome. These fever-reducing medications are not necessary for low grade fevers, which often improve on their own.

A cool water bath or cool towels applied to a person’s skin may help reduce body temperature. Individuals with a measured fever should protect against dehydration by drinking plenty of fluids. Any further treatment of a fever will vary depending on its cause and should be discussed with a medical provider.

What causes a fever?

Certain fever-producing substances, called pyrogens, can disrupt the body’s temperature-regulating system and cause a rise in the body’s set temperature. Pyrogens usually come from a source outside the body, such as viruses, bacteria, fungi, drugs, or toxins. The body can also produce it’s own endogenous pyrogens known as cytokines in response to inflammation and immune-mediated diseases. Other causes of fever include heat exhaustion, extreme sunburn, autoimmune disorders, hormone imbalances, or cancer. Determining the exact cause of a fever is difficult, but monitoring the measured temperatures and associated symptoms can ascertain its severity and allow for proper treatment.


Infections can be viral, bacterial, or fungal in origin and can all cause fevers. Monitoring the symptoms associated with your fever may allow for the diagnosis of the underlying condition.

Viral illness is among the most common causes of fevers in adults. Associated symptoms may include a runny nose, sore throat, cough, hoarseness, or muscle aches. Viruses may also cause nausea, vomiting, or diarrhea. Viral illnesses are not affected by antibiotics and will usually improve with time and at-home treatment, such as over the counter medications and increased hydration.

Bacterial infections can affect almost any organ system in the body, including the respiratory system, genitourinary system, reproductive system, gastrointestinal system, central nervous system, circulatory system, or the skin. Bacterial infections will usually persist for more than a few days and will often require a consultation with a medical provider for treatment with antibiotics. A more severe bacterial infection could require hospitalization and intravenous antibiotics.

Fungal infections can also affect any organ system but are treated with an anti-fungal medication. Fungal infections are often most difficult to diagnose and may require a biopsy for diagnosis and treatment.

  • Upper respiratory system infections affect the throat, ears, nose, and sinuses, causing sinus symptoms, headache, earache, cough, or sore throat with a fever.

  • Lower respiratory system infections, such as pneumonia and bronchitis, can also cause a fever with coughing, difficulty breathing, and sometimes chest pain.

  • Infection of the genitourinary system causes a urinary tract infection and is commonly associated with a fever along with a burning sensation when urinating, blood in the urine, the urge to urinate frequently, and back pain.

  • Infection of the reproductive system may cause a fever along with pelvic pain and discharge from the penis or vagina.

  • Gastrointestinal system infections can cause a fever with diarrhea, vomiting, abdominal pain, and sometimes blood in the stool. These symptoms may indicate an infection of the appendix, gallbladder, or liver and should be assessed by a medical provider.

  • Infection of the central nervous system can cause a fever with associated lethargy, headache, neck stiffness, confusion, or photophobia. These symptoms could indicate meningitis or a brain infection and should be evaluated immediately by a medical provider.

  • Infection of the circulatory system requires hospitalization and immediate treatment with intravenous antibiotics. When bacteria enter the bloodstream, it is known as sepsis, and is usually indicated by a high grade fever with generalized weakness, confusion, or altered mental status. Infection of a heart valve, known as endocarditis, can occur in people with prior heart surgeries or those who use intravenous drugs.

  • Skin infections result in redness, swelling, warmth, pus, or pain at the site of the infection. Skin infections often start off as an abscess, a swollen accumulation of pus, and progress to cellulitis, an infection to the soft tissues beneath the skin. Fevers may begin gradually with worsened skin infections and are an indication to seek medical evaluation.


Hormone Imbalances

Neurotransmitters and hormones help to regulate hypothalamic activity via a feedback mechanism. Imbalances in hormone and neurotransmitter levels can disrupt the temperature regulating functions of the hypothalamus and cause a fever. Hormone changes throughout the day or month can result in low-grade fevers in both men and women. Individuals taking hormone replacement therapy are more susceptible to hormone imbalances and should consult a medical provider if they have a fever.

New Medications

Fevers can also be caused by a new medication, an incidence known as drug fever. This may be a result of an allergic reaction to the medication or a preservative in the medication. It can also be a result of a disruption in the body’s thermoregulatory mechanism.

Antibiotics are most likely to increase the body’s temperature, specifically the beta-lactam and sulfonamide antibiotic classes. The onset of the fever is highly variable depending on the class of the fever-inducing drug; however, it is most commonly about 7-10 days after drug administration. Individuals who suspect they may have a drug fever should speak to a medical provider about possibly discontinuing the new medication.

Illegal Drugs

Illegal drugs can alter the function of the body’s temperature regulating system and cause a fever.

  • Amphetamines

  • MDMA (ecstasy)

  • Cocaine

  • Cannabis (marijuana)

  • Phencyclidine (PCP)


The specific increase in body temperature can depend on the quantity of the drug used. Individuals using illegal drugs, especially intravenous drugs, should seek medical evaluation when experiencing high grade fevers.

Environmental Causes

Exposure to high temperatures can cause a fever and accompanied dehydration. This may result in heat exhaustion or, in more serious cases, heat stroke. Associated symptoms of heat exhaustion include confusion, dark colored urine, dizziness, fainting, fatigue, headache, muscle aches, nausea, vomiting, pale skin, profuse sweating, and rapid heartbeat. Individuals who experience symptoms of heat exhaustion should immediately get out of the heat, rest, and drink plenty of fluids. Attempting to dry and reduce the fever with a cool shower, fans, or ice towels is recommended. If these measures do not provide relief within 15 minutes, individuals with heat exhaustion should seek emergency medical treatment in order to prevent heat stroke, a potentially deadly condition that occurs when your body temperature reaches 104°F or higher. Heatstroke is an emergent condition and should be evaluated by a medical provider immediately to prevent brain damage or damage to other vital organs.

Prevention of Fever

Since most fevers are caused by infection, taking steps to prevent an infection is the best way to prevent fevers.

  • Boost immunity. People can improve their overall immunity by leading a healthy lifestyle. Getting enough sleep, exercising frequently and eating a healthy diet can all help prevent disease by boosting the body’s immune system.

  • Avoid sick people. Whenever possible, it helps to avoid contact with people who are ill.

  • Get vaccinated. Making sure immunizations are up-to-date in both children and adults, especially if traveling to another country.


Environmental fevers can also be avoided by staying well hydrated, wearing cool clothing, and seeking shelter when in the heat. Be sun smart!

Who is at Risk?

Fevers can impact anyone, from infants to the elderly. However, there are some individuals who are more susceptible to infection and therefore fevers.

Age. Infants and children up to age 4 and adults over the age of 65 are particularly vulnerable to fevers.


Certain medical conditions can prevent the immune system from functioning, making it easier for fever-inducing infections to invade the body. Causes of a weakened immune system include the following:

  • Cancer and cancer treatments

  • Organ transplant immunosuppressive medication

  • Long term steroid therapy

  • Human immunodeficiency virus (HIV)

  • Splenectomy

  • Lupus

  • Diabetes

  • Heart disease


When Should I Call a Doctor?

Remember that a person’s body temperature can vary throughout the day and in different environments. An elevation in body temperature is not necessarily a cause for concern. However, adults with a temperature greater than 103°F (39.4°C) or a fever that persists for more than a few days, despite over the counter treatments and increased hydration, should seek evaluation from a medical provider. Individuals with a history or diagnosis of diabetes, HIV, heart disease, lupus, or cancer who become febrile should consult their doctor. Individuals who take immunosuppressant drugs or recently started taking a new medication should also seek medical care for a fever.

Fever in Infants and Children

Fevers are generally of greater concern in infants and young children than in adults. Any child below 3 months of age who has a rectal temperature greater than 100.4°F should be evaluated by a medical provider.

In children above the age of 3 months, a fever may be the body’s way of building up the immune system and preventing infection. If your child has a fever but remains responsive, playful, and is drinking fluids, there is likely no cause for concern. Fever-reducing medications, such as acetaminophen (Tylenol), ibuprofen (Advil or Motrin), and naproxen (Aleve) can be given to children to reduce discomfort. Children and adolescents below the age of 19 should not be given Aspirin as it is linked to a potentially deadly condition known as Reye’s syndrome.


If your child becomes irritable, behaves unusually, or appears listless, consult a medical provider. Fevers in children associated with vomiting, severe headache, or abdominal pain should also be evaluated by a medical provider.

Acetaminophen (Tylenol) Dosage Table

Child's Weight (pounds) 6-11 12-17 18-23 24-35 36-47 48-59 60-71 72-95 96+ lbs
Infant Drops 80 mg/0.8 ml 0.4 0.8 1.2 1.6 2.4 -- -- -- -- ml
Syrup: 160 mg/5 mL (1 tsp) 1.25 2.5 3.75 5 7.5 10 12.5 15 20 ml
Syrup: 160 mg/1 teaspoon -- 1/2 3/4 1 1 1/2 2 2 1/2 3 4 tsp
Chewable 80 mg tablets -- -- 1 1/2 2 3 4 5 6 8 tabs
Chewable 160 mg tablets -- -- -- 1 1 1/2 2 2 1/2 3 4 tabs
Adult 325 mg tablets -- -- -- -- -- 1 1 1 1/2 2 tabs
Adult 500 mg tablets -- -- -- -- -- -- -- 1 1 tabs

Ibuprofen (Advil, Motrin) Dosage Table

Child's Weight (pounds) 12-17 18-23 24-35 36-47 48-59 60-71 72-95 96+ lbs
Infant Drops 50mg/1.25 ml 1.25 1.875 2.5 3.75 5 -- -- -- ml
Liquid 100 mg/ 1 teaspoon (tsp) ½ ¾ 1 2 3 4 tsp
Liquid 100 mg/ 5 milliliters (ml) 2.5 4 5 7.5 10 12.5 15 20 ml
Chewable 50 mg. tablets -- -- 2 3 4 5 6 8 tabs
Junior-strength 100 mg tablets -- -- -- -- 2 3 4 tabs
Adult 200 mg. tablets -- -- -- -- 1 1 2 tabs

Emergency Warning Signs

Certain conditions associated with a fever can be life threatening and should be evaluated immediately by a medical provider in an emergency room setting. Individuals should seek emergency medical care if a fever is associated with any of the following symptoms:

  • Confusion

  • Stiff neck

  • Severe headache

  • Rash

  • Red, hot, or swollen area of the skin

  • Leg swelling

  • Difficulty swallowing

  • Difficulty breathing

  • Chest pain

  • Excessive vomiting

  • Abdominal pain

  • Blood in stool

This page offers general health information to facilitate discussion with your telehealth provider. You must not rely on the information on our website as an alternative to medical advice from your doctor or other professional healthcare provider.

If this is a medical emergency, please call 911. For mental health emergencies, call 988.

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