Fever is an elevated body temperature, primarily in response to an infection. Although 98.6° F (37° C) is considered normal, body temperature varies throughout the day. 

Despite people worrying that fever can cause harm, the typical temporary elevations in body temperature ranging from 100.4° to 104° F caused by most short-lived (acute) infections are well-tolerated by healthy adults. However, extreme temperature elevation (typically more than 105.8° F or 41° C) may be damaging as it may cause malfunction and, ultimately, failure of most organs. Such extreme elevation usually results from very severe infection (such as sepsis, malaria, or meningitis) but is more typically caused by heatstroke or certain drugs.

In adults, a fever that lasts four days or less may be due to an infectious cause. A noninfectious cause is suspected in the fever that lasts a long time or returns. Many cancers and inflammatory disorders may also cause fever.

Drugs like beta-lactam antibiotics (such as penicillin) and sulfa drugs can trigger a fever. Some drugs can cause an extremely high temperature like certain illicit drugs (such as cocaine, amphetamines, or phencyclidine), anesthetics, and antipsychotic drugs.

The risk factors for fever include the persons health status, age, certain occupations, use of certain medical procedures and drugs, and exposure to infections.

In people with an acute fever, certain signs and characteristics may be a warning, like a change in mental function, such as confusion; headache, stiff neck, or both; petechiae on the skin; low blood pressure; rapid heart rate; dyspnea; temperature >104° F (40° C) or <95° F (35° C); recent travel to an endemic area; current use of immunosuppressants.

Testing is usually unnecessary for fever except for people at risk of COVID-19, those exposed to a vector, and those who have recently been in an endemic area. It may also be needed if otherwise healthy people have findings suggesting a particular disorder.

Fever is managed by treating its cause. acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs) are the most effective and widely used antipyretics. Other cooling measures (such as cooling with a tepid water mist and using cooling blankets) are required only if the temperature is about 106° F (41.1° C) or higher. 

Fever can be challenging in older people since their body may not respond similar to the younger people. Frail older people have fewer chances of getting a fever by infection. Diagnosis of fever in older people is similar to the younger adults, except that the doctors may recommend some additional tests.

Bush LM. Fever in Adults. MSD manual. 2022