Fever / Pyrexia

Anti-infection defense mechanism that appears with body temperature exceeding the normal range due to an increase in the body's temperature set point in the hypothalamus.
The increase in set point triggers increased muscle contractions and causes a feeling of cold or chills. This results in greater heat production and efforts to conserve heat. When the set point temperature returns to normal, a person feels hot, becomes flushed, and may begin to sweat.
May rarely trigger a febrile seizure. This is more common in young children.
Fevers do not typically go higher than 41 to 42 °C (106 to 108 °F).
A fever can be caused by many medical conditions ranging from non-serious to life-threatening. This includes viral, bacterial, and parasitic infections—such as influenza, the common cold, meningitis, urinary tract infections, appendicitis, Lassa fever, COVID-19, and malaria.
Non-infectious causes include vasculitis, deep vein thrombosis, connective tissue disease, side effects of medication or vaccination, and cancer.
Treatment of associated pain and inflammation, may be useful and help a person rest.
Medications (e.g. ibuprofen or paracetamol) may help with this as well as lower temperature.
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.
One of the most common medical signs.
It is part of about 30% of healthcare visits by children and occurs in up to 75% of adults who are seriously sick.
A fever is usually accompanied by sickness behavior, which consists of lethargy, depression, loss of appetite, sleepiness, hyperalgesia, dehydration,and the inability to concentrate.
Sleeping with a fever can often cause intense or confusing nightmares, commonly called "fever dreams".
Mild to severe delirium (which can also cause hallucinations) may also present itself during high fevers.
Clinically, it is important to distinguish between fever and hyperthermia as hyperthermia may quickly lead to death and does not respond to antipyretic medications. The distinction may however be difficult to make in an emergency setting, and is often established by identifying possible causes.
Behavior

Various patterns of measured patient temperatures have been observed, some of which may be indicative of a particular medical diagnosis:

Continuous fever, where temperature remains above normal and does not fluctuate more than 1 °C in 24 hours (e.g. in bacterial pneumonia, typhoid fever, infective endocarditis, tuberculosis, or typhus).
Intermittent fever is present only for a certain period, later cycling back to normal (e.g., in malaria, leishmaniasis, pyemia, sepsis, or African trypanosomiasis).
Remittent fever, where the temperature remains above normal throughout the day and fluctuates more than 1 °C in 24 hours (e.g., in infective endocarditis or brucellosis).
Pel–Ebstein fever is a cyclic fever that is rarely seen in patients with Hodgkin's lymphoma.
Undulant fever, seen in brucellosis.
Typhoid fever is a continuous fever showing a characteristic step-ladder pattern, a step-wise increase in temperature with a high plateau.

Among the types of intermittent fever are ones specific to cases of malaria caused by different pathogens. These are:

Quotidian fever, with a 24-hour periodicity, typical of malaria caused by Plasmodium knowlesi (P. knowlesi);
Tertian fever, with a 48-hour periodicity, typical of later course malaria caused by P. falciparum, P. vivax, or P. ovale;
Quartan fever, with a 72-hour periodicity, typical of later course malaria caused by P. malariae.

Causes

Fever is a common symptom of many medical conditions:

Infectious disease such as:

Immunological diseases such as:

Tissue destruction

As a result of cerebral bleeding
Crush syndrome
Hemolysis
Infarction
Rhabdomyolysis
Surgery
Etc.

Cancers, particulary blood cancers such as:

Metabolic disorders such as:

Inherited metabolic disorder such as:

Fabry disease

Adult and pediatric manifestations for the same disease may differ; for instance, in COVID-19, one metastudy describes 92.8% of adults versus 43.9% of children presenting with fever.
In addition, fever can result from a reaction to an incompatible blood product.
Assist the healing process in ways, including:

Increased mobility of leukocytes;
Increased proliferation of T cells.
Enhanced leukocyte phagocytosis;
Decreased endotoxin effects;

Temperature is regulated in the hypothalamus. The trigger of a fever, called a pyrogen, results in the release of prostaglandin E2 (PGE2). 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. There are four receptors in which PGE2 can bind (EP1-4), with a previous study showing the EP3 subtype is what mediates the fever response. When the set point is raised, the body increases its temperature through both active generation of heat and retention of heat. 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.
If these measures are insufficient to make the blood temperature in the brain match the new set point in the hypothalamus, the brain orchestrates heat effector mechanisms via the autonomic nervous system or primary motor center for shivering. These may be:

Increased heat production by increased muscle tone, shivering (muscle movements to produce heat) and release of hormones like epinephrine; and
Prevention of heat loss, e.g., through vasoconstriction.

Fever / Pyrexia

Behavior

Causes