What is a fever?

A fever is a higher than normal body temperature. It is often associated with an infection or illness. A fever has long been considered an important sign of illness and has historically been used for diagnosis and treatment. Infants and young children often have a higher fever than adults, but generally a fever is anything over 100°F (37.8°C).


A fever is not an illness in itself, but rather a symptom of another condition. A fever is usually caused by an infection of some kind, either viral, bacterial, or fungal. Other causes include:

  • body reactions to medications
  • inflammation
  • immune system response to foreign bodies, such as transplanted organs
  • an allergic reaction
  • malignancy
  • autoimmune disorders, such as rheumatoid arthritis
  • endocrine disorders, such as an overactive thyroid


Most of the time, a fever resolves on its own without any medical intervention. Treating a fever usually involves administering medication to reduce the fever if it is high or lasts longer than a few days. Medications used to reduce a fever include acetaminophen and ibuprofen. In some cases, if the cause of the fever is known, the underlying condition is treated directly.


If a high fever is not treated, it can lead to dehydration or other complications. It is important to seek medical attention if the fever spikes to 103°F (39.4°C) or higher, if the fever has lasted more than three days, or if the fever is accompanied by other symptoms, such as chills, rash, or vomiting. It is also important to seek medical attention if the person with the fever is an infant, an elderly adult, or an individual with an underlying medical condition.