What Is a Fever?
A fever is a body temperature that’s higher than is considered normal. It’s also called a high temperature, hyperthermia, or pyrexia, and it’s usually a sign that your body is working to keep you healthy from an infection. Normal body temperatures are different for everyone, but they lie within the range of 97 to 99. A temperature of 100.4 or higher is considered a fever.
A part of your brain called the hypothalamus controls your body temperature. In response to an infection, illness, or some other cause, the hypothalamus may reset the body to a higher temperature. So when a fever comes on, it’s a sign that something is going on in your body.
Fevers themselves generally aren’t dangerous, but you should check in with your doctor if:
- An adult’s temperature is 103 or higher
- A very young infant (under 3 months) has a rectal temperature 100.4 or higher (call your doctor or go to an emergency room immediately)
- A 3-6-month-old has a higher than normal rectal temperature and is also irritable or sleepy (call your doctor right away)
- A 3-6-month-old has a 102 or higher rectal temperature
- A 6-24-month-old has a fever higher than 102 for more than a day or with other symptoms such as a cough or diarrhea
- A child older than 2 has a fever that comes with rash, real discomfort, irritability, listlessness, headache, stiff neck, or repeated diarrhea or vomiting
- An infant or child has a seizure
- Any temperature over 104 in a child, which could cause a seizure
- Any fever that starts after someone has been in hot temperatures, which could be a sign of heat stroke
- The fever doesn’t go down after taking over-the-counter medications such as ibuprofen in the appropriate doses
- You’ve been in contact with someone who has COVID-19
Fevers are signs of some sort of illness or infection. When you have, you may also notice these symptoms:
A fever can be a sign of several health conditions, which may or may not need medical treatment.
- Infections of the ear, lung, skin, throat, bladder, or kidney
- Heat exhaustion
- Conditions that cause inflammation, such as rheumatoid arthritis
- Side effects of medications
- Vaccines and immunizations
- Blood clots
- Autoimmune conditions such as lupus and inflammatory bowel disease (IBS)
- Hormone disorders such as hyperthyroidism
- Illegal drugs such as amphetamines and cocaine
- Teething in babies can cause a mild, low-grade fever (not over 101 degrees)
Although a fever is easy to measure with a thermometer, finding its cause can be hard. Besides a physical exam, your doctor will ask about symptoms and conditions, medications, and if you've recently traveled to areas with infections or have other infection risks. A malaria infection, for example, may cause a fever that typically comes back. Some areas of the U.S. are hot spots for infections such as Lyme disease and Rocky Mountain spotted fever.
Your doctor may ask if you have been around someone with COVID-19 or have any other symptoms of COVID-19.
Sometimes, you may have a "fever of unknown origin." In such cases, the cause could be an unusual or not obvious condition such as a chronic infection, a connective tissue disorder, cancer, or another problem.
Fever is usually associated with physical discomfort, and most people feel better when a fever is treated. But depending on your age, physical condition, and the underlying cause of your fever, you may or may not require medical treatment for the fever alone. Many experts believe that fever is a natural bodily defense against infection. There are also many non-infectious causes of fever.
The most common treatments for fever include over-the-counter drugs such as acetaminophen and nonsteroidal anti-inflammatory drugs such as ibuprofen and naproxen. Children and teens should not take aspirin because it's linked to a condition called Reye’s syndrome.
Ways to lower a fever at home include: