[5][6][12][7] There is no single agreed-upon upper limit for normal temperature: sources use values ranging between 37.2 and 38.3 °C (99.0 and 100.9 °F) in humans.

[3] When the set point temperature returns to normal, a person feels hot, becomes flushed, and may begin to sweat.

[13] This includes viral, bacterial, and parasitic infections—such as influenza, the common cold, meningitis, urinary tract infections, appendicitis, Lassa fever, COVID-19, and malaria.

[13][14] Non-infectious causes include vasculitis, deep vein thrombosis, connective tissue disease, side effects of medication or vaccination, and cancer.

"[2][20] A fever is usually accompanied by sickness behavior, which consists of lethargy, depression, loss of appetite, sleepiness, hyperalgesia, dehydration,[21][22] and the inability to concentrate.

[37] For example, the temperature rises in healthy people when they exercise, but this is not considered a fever, as the set point is normal.

[7]: 103 Various patterns of measured patient temperatures have been observed, some of which may be indicative of a particular medical diagnosis: Among the types of intermittent fever are ones specific to cases of malaria caused by different pathogens.

[55] Because of the lack of infection-fighting neutrophils, a bacterial infection can spread rapidly; this fever is, therefore, usually considered to require urgent medical attention.

In contrast, hyperthermia involves body temperature rising above its set point due to outside factors.

[7][60] The high temperatures of hyperpyrexia are considered medical emergencies, as they may indicate a serious underlying condition or lead to severe morbidity (including permanent brain damage), or to death.

[79] Studies using warm-blooded vertebrates suggest that they recover more rapidly from infections or critical illness due to fever.

PGE2 in turn acts on the hypothalamus, which creates a systemic response in the body, causing heat-generating effects to match a new higher temperature set point.

Peripheral vasoconstriction both reduces heat loss through the skin and causes the person to feel cold.

Norepinephrine increases thermogenesis in brown adipose tissue, and muscle contraction through shivering raises the metabolic rate.

Hyperthermia is usually the result of an excessively hot environment (heat stroke) or an adverse reaction to drugs.

[7][verification needed] In infants, the autonomic nervous system may also activate brown adipose tissue to produce heat (non-shivering thermogenesis).

[89] In the presence of an infectious agent, such as bacteria, viruses, viroids, etc., the immune response of the body is to inhibit their growth and eliminate them.

[97] These cytokine factors are released into general circulation, where they migrate to the brain's circumventricular organs where they are more easily absorbed than in areas protected by the blood–brain barrier.

These proteins produce a cyclooxygenase which induces the hypothalamic production of PGE2 which then stimulates the release of neurotransmitters such as cyclic adenosine monophosphate and increases body temperature.

In general, these pyrogens, including bacterial cell wall products, may act on Toll-like receptors in the hypothalamus and elevate the thermoregulatory setpoint.

[101] An example of a class of exogenous pyrogens are bacterial lipopolysaccharides (LPS) present in the cell wall of gram-negative bacteria.

It is presumed that the innervation from the POA to the PVN mediates the neuroendocrine effects of fever through the pathway involving pituitary gland and various endocrine organs.

[105] Damage to the brain generally does not occur until temperatures reach 40.0 °C (104.0 °F), and it is rare for an untreated fever to exceed 40.6 °C (105.1 °F).

In theory, these critically ill patients and those faced with additional physiologic stress may benefit from fever reduction, but the evidence on both sides of the argument appears to be mostly equivocal.

[116] Additionally, aspirin is not recommended in children and young adults (those under the age of 16 or 19 depending on the country) due to the risk of Reye's syndrome.

This term fell out of use in the early 20th century, and the symptoms it referred to are now thought to have been caused mainly by various minor viral respiratory infections.

[126] They are also afraid of harmless side effects like febrile seizures and dramatically overestimate the likelihood of permanent damage from typical fevers.

[126] The underlying problem, according to professor of pediatrics Barton D. Schmitt, is that "as parents we tend to suspect that our children's brains may melt.

For instance, some species of grasshopper will thermoregulate to achieve body temperatures that are 2–5 °C higher than normal in order to inhibit the growth of fungal pathogens such as Beauveria bassiana and Metarhizium acridum.