Causes of Fevers and When to Worry

Fever isn't always cause for concern

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A fever is a temporary increase in body temperature. It is usually a sign that you have an illness. A temperature of 98.6 degrees Fahrenheit is generally considered normal, although some studies have found that normal body temperature can range from 97 degrees to 99 degrees. If you think you or your child might have a fever, you can confirm this with a thermometer.

A low-grade fever below 100.4 degrees Fahrenheit is usually not a problem. If you have a high-grade fever (above 100.4 degrees Fahrenheit), talk to your healthcare provider.

This article discusses low-grade and high-grade fevers, what causes them, and when to see a healthcare provider.

What Are the Symptoms of Fever?

A fever can develop quickly or slowly. It can be intermittent or constant. The pattern often depends on the cause. Fevers can range from what is considered low-grade (below 100.4 degrees) to a high-grade fever above 100.4 degrees.

While a high body temperature can cause symptoms, the underlying medical condition that caused the fever can also cause symptoms, making you feel very sick. You may notice that the intensity of certain symptoms correspond with your fever, while others do not change along with your fevers.

Common symptoms of fevers include:

Depending on the cause of the fever, you may experience associated symptoms such as a sore throat, runny nose, earache, stomach upset, rash, coughing, muscle aches, or pain with urination.

Complications of Fever

An untreated fever can be associated with serious health issues, especially in young children. High-grade fevers are far more likely to cause major problems than low-grade fevers.

Complications of fevers include:

  • Dehydration: Fevers are associated with fatigue, decreased fluid intake, and sweating—all of which can lead to dehydration.
  • Confusion: You may have diminished attention and confusion when you have a fever.
  • Hallucinations: A very high fever can cause a person to experience a fluctuating dream-like state, which can result in confusing hallucinations, especially when you aren't sure if you are awake or asleep.
  • Loss of consciousness: In some situations, especially with dehydration, a person can lose consciousness due to a fever.
  • Heat stroke: Having a high internal body temperature can have the same effects as a heat stroke, in which a person is exposed to a high outside temperature.
  • Febrile seizures: Body temperature changes affect the way proteins and neurotransmitters function in the body, potentially causing a sudden generalized tonic conic seizure. This complication is more common among young babies.

A fever is also called pyrexia.

Causes of Fevers
Verywell / Jiaqi Zhou

What Are the Main Causes of Fever?

Infections are the most common cause of a fever. There are a number of other medical conditions that cause fevers as well. Fevers are caused by an inflammatory reaction of the body that helps fight infections and illnesses.

Common infections that trigger a fever include:

  • Malaria: A parasitic infection that causes cyclic fevers. It is spread by mosquitos.
  • Q fever: A bacterial infection that causes high-grade fevers. It is transmitted to humans from animals such as cattle and sheep.
  • Yellow fever: A viral infection that causes prolonged high fevers and liver damage. It is spread by mosquitoes.
  • Scarlet fever: An infection that causes high fevers, a rash, and tongue swelling. It is caused by group A streptococcus bacterium the same bacteria that causes strep throat.
  • Dengue fever: A viral infection that causes internal bleeding and fevers. It is transmitted by mosquitoes and caused by the viruses Aedes aegypti or Aedes albopictus.

Most infections can cause a fever. In general, bacterial infections and parasitic infections are the most likely to cause high-grade fevers. Infections that spread throughout the body, causing sepsis, typically cause a high fever.

Non-Infectious Causes of Fevers

Inflammatory conditions that involve the immune system can trigger fevers, even when they do not involve an infection.

Illnesses that cause fevers include:

  • Cancer
  • Autoimmune disorders, like lupus, sarcoidosis, and arthritis
  • Non-infectious encephalitis (inflammation of the brain)
  • A ruptured appendix (which often begins as an infection)
  • Heat exhaustion and heat stroke

Certain medications can cause a fever, including cephalosporine and Aldomet (methyldopa). Antidepressants can cause serotonin syndrome, a dangerous reaction consisting of muscle rigidity and fevers. Drugs of abuse, such as methamphetamine, can cause a fever too.

Overdressing, which is common with infants that are wrapped in blankets or too much clothing, can cause a fever as well.

How A Fever Starts

When you have an infection, one of the ways that your immune system responds to germs is by raising your body temperature, which makes it harder for the germs to survive. Immune cells that are activated in response to infections release immune proteins called cytokines that raise the body temperature.

Some other illnesses that trigger inflammation can cause fevers due to the involvement of similar immune mechanisms.

When should you worry about a fever?

A low-grade fever (below 100.4 degrees) is usually not a cause for concern. A baby younger than 3 months with a fever above 100.4 degrees should be seen by a healthcare provider right away.

In adults, a fever higher than 104 degrees needs prompt medical evaluation. You should also see a healthcare provider if you have other concerning symptoms such as:

  • Stiff neck
  • Trouble breathing
  • Loss of consciousness
  • Severe pain
  • Confusion
  • Swelling or inflammation

How Is Fever Diagnosed?

There are several ways to check your body temperature or your child's body temperature. Each of the methods requires a device that is specifically designed for that method of checking body temperature.

The results may vary by about a degree depending upon the part of the body that you are testing. Whichever method you use, you will get a good general idea of your temperature. But it's important to follow the instructions and the benchmarks for the method that you are using.

Ways of checking body temperature include:

  • Oral (by mouth): The most commonly used thermometers check temperatures by mouth. This method requires that the thermometer is held underneath the tongue for about three minutes before reading the temperature.
  • Tympanically (in the ear): A tympanic thermometer is held at the opening of the ear. To avoid any injury, the thermometer must be placed at the entrance of the ear canal, and should not be pushed deep into the canal. Pain or discomfort is a sign that the thermometer is positioned too deep in the ear canal or it can be a sign of an ear injury. After about three minutes, you can read the temperature on the thermometer.
  • Temporally (on the forehead or temple): This type of temperature measurement uses a device that is placed on the forehead. The device may change colors to correspond to body temperature or may give a digital read. This is the least invasive and least accurate method of checking the body temperature.
  • Axillary: You can check the axillary temperature by placing the thermometer underneath the armpit. This is not a common method, particularly because children tend to move around and squirm during the process.
  • Rectally: This method is typically used for babies because it is the most accurate. Babies might not be able to cooperate with other methods. As with the other methods, it is necessary to leave the thermometer in place for several minutes to get an accurate reading.

Clean the thermometer, preferably with an alcohol pad, between each use. It is a good idea to repeat a temperature measure to confirm whether you or your child has a fever.

Diagnostic Tests

If you have a fever, your healthcare provider may run some tests to determine the cause. You may need blood tests, which can identify inflammatory cells, and may often differentiate between infections and inflammatory diseases.

A blood culture, urine culture, or throat culture can be used to grow an infectious organism in a laboratory setting, helping direct treatment.

Sometimes, imaging tests are needed if there is a concern that you could have an abscess (an enclosed infection), a ruptured appendix, or cancer.

How to Treat a Fever

Fevers can usually be treated with over-the-counter (OTC) fever-reducing medications, such as Advil or Motrin (ibuprofen), or Tylenol (acetaminophen). In general, these medications can lower your body temperature and make you more comfortable for about four hours. Some of the other symptoms that you may be experiencing—such as a sore throat, nausea, or a rash—are not likely to improve based on the treatment of your fever.

It is a good idea to check with your healthcare provider and to see how long you should wait before going in to be seen. For example, your healthcare provider may tell you to come in if you still have fevers after a week. If you have a condition such as cancer, your healthcare provider may want to to be seen even after a day or two of fevers.

Some strategies that can help reduce fever include staying hydrated and placing a cool towel on the arms or forehead for comfort.


Children may have a fever for just a day, and then feel great the next day. Many childhood infections resolve on their own fairly quickly. Nevertheless, it is still a good idea to talk to your child's pediatrician to get some guidelines about when you should bring in your child and how long you should keep them home from school.

If you are treating a child with a fever, there are a few important things to keep in mind:

  • Never give aspirin or baby aspirin to a child who is under 18 years old. It can cause a serious illness called Reye's Syndrome.
  • Children under 6 months old should not take Motrin or Advil (ibuprofen).
  • Children under 2 months old should not be given any type of fever-reducing medication
  • Never put a child or anyone else in an ice or alcohol bath to bring down the fever. This is unnecessary and it can be dangerous, making the body temperature drop too quickly.


Fevers are particularly concerning in young babies. This is because babies have fragile temperature regulation and can experience serious effects as a result of a fever. Young babies should get medical attention for high temperatures.

  • Infants under 3 months old: Call your healthcare provider or seek medical attention right away for any temperature over 100.4 degrees Fahrenheit.
  • Babies ages 3 months to 3 years: Call your healthcare provider or get prompt medical attention for a temperature over 102.2 degrees Fahrenheit.

Using alcohol wipes, rubbing alcohol, or an alcohol bath had been used as an at-home method for reducing fevers. This is dangerous and it is not recommended. Alcohol can lead to dehydration, and it can be toxic—never use alcohol as a way to manage fevers.


Fevers are the body's way of fighting off infections. A fever is a sign that you have an illness that needs to be taken care of. A normal body temperature ranges from 97 degrees to 99 degrees. A fever below 100.4 degrees is considered low-grade; anything above that is considered high-grade.

Most of the time, fever-inducing infections are not serious and improve on their own after a few days of rest. However, a fever can be a sign of a more serious problem, so be sure to call your healthcare provider if you have a high-grade fever, a prolonged fever, or if your young baby has a fever.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Kristina Herndon, RN
Kristina Herndon, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.