A fever -- also known as a high fever or a high temperature -- is not by itself an illness. It's usually a symptom of an underlying condition, most often an infection.
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.
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Fever is generally not considered dangerous, but hyperthermia can cause dangerous rises in body temperature. This can be due to an extreme temperature associated with heat injury such as heat stroke, side effects of certain medications or illicit drugs, and stroke. With hyperthermia, the body is no longer able to control body temperature.
In children with fever, accompanying symptoms such as lethargy, fussiness, poor appetite, sore throat, cough, ear pain, and diarrhea are important to relay to your doctor.
According to the American Academy of Pediatrics, if you have an infant younger than 4 months old with a rectal temperature of 100.4 F or above, you should immediately call your doctor or go to an emergency room because it could be a sign of a potentially life-threatening infection. Also call your doctor or go to the emergency room if any child has a fever above 104 F. High fever can cause seizures in young children.
Call your doctor right away if your child has a fever and:
Looks very sick
Is drowsy or very fussy
Has a weakened immune system or other medical problems
Has a seizure
Has other symptoms such as rash, sore throat, headache, stiff neck, or earache
Call the doctor if the fever lasts more than 1 day in a child less than 2 years old or lasts more than 3 days in a child age 2 or older.
Causes of Fever
A part of the brain called the hypothalamus controls body temperature, which usually varies throughout the day from the normal temperature of 98.6 F.
In response to an infection, illness, or some other cause, the hypothalamus may reset the body to a higher temperature.
Although the most common causes of fever are common infections such as colds and gastroenteritis, other causes include:
Although a fever is easy to measure, determining 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 be have a fever that typically recurs. Some areas of the U.S. are hotspots for infections such as Lyme disease and Rocky Mountain spotted fever.
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.