Fever, also referred to as pyrexia, is defined as having a temperature above the normal range due to an increase in the body's temperature set point.[5][6][7] There is not a single agreed-upon upper limit for normal temperature with sources using values between 37.2 and 38.3 °C (99.0 and 100.9 °F) in humans.[1][7][8] The increase in set point triggers increased muscle contractions and causes a feeling of cold or chills.[2] This results in greater heat production and efforts to conserve heat.[3] When the set point temperature returns to normal, a person feels hot, becomes flushed, and may begin to sweat.[3] Rarely a fever may trigger a febrile seizure, with this being more common in young children.[4] Fevers do not typically go higher than 41 to 42 °C (105.8 to 107.6 °F).[6]

A fever can be caused by many medical conditions ranging from non-serious to life-threatening.[12] This includes viral, bacterial, and parasitic infections—such as influenza, the common cold, meningitis, urinary tract infections, appendicitis, COVID-19, and malaria.[12][13] Non-infectious causes include vasculitis, deep vein thrombosis, connective tissue disease, side effects of medication or vaccination, and cancer.[12][14] It differs from hyperthermia, in that hyperthermia is an increase in body temperature over the temperature set point, due to either too much heat production or not enough heat loss.[1]

Treatment to reduce fever is generally not required.[2][9] Treatment of associated pain and inflammation, however, may be useful and help a person rest.[9] Medications such as ibuprofen or paracetamol (acetaminophen) may help with this as well as lower temperature.[9][10] Children younger than three months require medical attention, as might people with serious medical problems such as a compromised immune system or people with other symptoms.[15] Hyperthermia does require treatment.[2]

Fever is one of the most common medical signs.[2] It is part of about 30% of healthcare visits by children[2] and occurs in up to 75% of adults who are seriously sick.[11] While fever evolved as a defense mechanism, treating fever does not appear to worsen outcomes.[16][17] Fever is often viewed with greater concern by parents and healthcare professionals than is usually deserved, a phenomenon known as fever phobia.