A fever is usually accompanied by sickness behavior, which consists of lethargy, depression, anorexia, sleepiness, hyperalgesia, and the inability to concentrate.

A wide range for normal temperatures has been found. Central temperatures, such as rectal temperatures, are more accurate than peripheral temperatures. Fever is generally agreed to be present if the elevated temperature is caused by a raised set point and:

• Temperature in the anus (rectum/rectal) is at or over 37.5–38.3 °C (99.5–100.9 °F)
• Temperature in the mouth (oral) is at or over 37.7 °C (99.9 °F)
• Temperature under the arm (axillary) or in the ear (tympanic) is at or over 37.2 °C (99.0 °F)

In healthy adults, the range of normal, healthy temperatures for oral temperature is 33.2–38.2 °C (91.8–100.8 °F), for rectal it is 34.4–37.8 °C (93.9–100.0 °F), for tympanic membrane (the ear drum) it is 35.4–37.8 °C (95.7–100.0 °F), and for axillary (the armpit) it is 35.5–37.0 °C (95.9–98.6 °F). Harrison's principles of internal medicine defines a fever as a morning oral temperature of >37.2 °C (>98.9 °F) or an afternoon oral temperature of >37.7 °C (>99.9 °F) while the normal daily temperature variation is typically 0.5 °C (0.9 °F).

Normal body temperatures vary depending on many factors, including age, sex, time of day, ambient temperature, activity level, and more. A raised temperature is not always a fever. For example, the temperature of a healthy person rises when he or she exercises, but this is not considered a fever, as the set-point is normal. On the other hand, a "normal" temperature may be a fever, if it is unusually high for that person. For example, medically frail elderly people have a decreased ability to generate body heat, so a "normal" temperature of 37.3 °C (99.1 °F) may represent a clinically significant fever.

Types

The pattern of temperature changes may occasionally hint at the diagnosis:

• Continuous fever: Temperature remains above normal throughout the day and does not fluctuate more than 7002274149999999999?1 °C in 24 hours, e.g. lobar pneumonia, typhoid, meningitis, urinary tract infection, or typhus. Typhoid fever may show a specific fever pattern (Wunderlich curve of typhoid fever), with a slow stepwise increase and a high plateau. (Drops due to fever-reducing drugs are excluded.)
• Intermittent fever: The temperature elevation is present only for a certain period, later cycling back to normal, e.g. malaria, kala-azar, pyaemia, or septicemia. Following are its types:
  • Quotidian fever, with a periodicity of 24 hours, typical of Plasmodium falciparum or Plasmodium knowlesi malaria
  • Tertian fever (48-hour periodicity), typical of Plasmodium vivax or Plasmodium ovale malaria
  • Quartan fever (72-hour periodicity), typical of Plasmodium malariae malaria.
• Remittent fever: Temperature remains above normal throughout the day and fluctuates more than 7002274149999999999?1 °C in 24 hours, e.g., infective endocarditis, brucellosis.
• Pel-Ebstein fever: A specific kind of fever associated with Hodgkin's lymphoma, being high for one week and low for the next week and so on. However, there is some debate as to whether this pattern truly exists.

A neutropenic fever, also called febrile neutropenia, is a fever in the absence of normal immune system function. 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. This kind of fever is more commonly seen in people receiving immune-suppressing chemotherapy than in apparently healthy people.

Febricula is an old term for a low-grade fever, especially if the cause is unknown, no other symptoms are present, and the patient recovers fully in less than a week.

Hyperpyrexia

Hyperpyrexia is an extreme elevation of body temperature which, depending upon the source, is classified as a core body temperature greater than or equal to 40.0 or 41.5 °C (104.0 or 106.7 °F). Such a high temperature is considered a medical emergency, as it may indicate a serious underlying condition or lead to problems including permanent brain damage, or death. The most common cause of hyperpyrexia is an intracranial hemorrhage.^[32] Other possible causes include sepsis, Kawasaki syndrome,^[35] neuroleptic malignant syndrome, drug overdose, serotonin syndrome, and thyroid storm.^[33]

Infections are the most common cause of fevers, but as the temperature rises other causes become more common.^[33] Infections commonly associated with hyperpyrexia include roseola, measles and enteroviral infections.^[35] Immediate aggressive cooling to less than 38.9 °C (102.0 °F) has been found to improve survival.^[33] Hyperpyrexia differs from hyperthermia in that in hyperpyrexia the body's temperature regulation mechanism sets the body temperature above the normal temperature, then generates heat to achieve this temperature, while in hyperthermia the body temperature rises above its set point due to an outside source.^[34]

Hyperthermia

Hyperthermia is an example of a high temperature that is not a fever. It occurs from a number of causes including heatstroke, neuroleptic malignant syndrome, malignant hyperthermia, stimulants such as substituted amphetamines and cocaine, idiosyncratic drug reactions, and serotonin syndrome.^[36][37]

Differential diagnosis

Fever is a common symptom of many medical conditions:

• Infectious disease, e.g., influenza, primary HIV infection, malaria, Ebola, infectious mononucleosis, gastroenteritis, Lyme disease, Dengue
• Various skin inflammations, e.g., boils, abscess
• Immunological diseases, e.g., lupus erythematosus, sarcoidosis, inflammatory bowel diseases, Kawasaki disease, Still disease, Horton disease, granulomatosis with polyangiitis, autoimmune hepatitis, relapsing polychondritis^[38]
• Tissue destruction, which can occur in hemolysis, surgery, infarction, crush syndrome, rhabdomyolysis, cerebral bleeding, etc.
• Reaction to incompatible blood products
• Cancers, most commonly kidney cancer and leukemia and lymphomas
• Metabolic disorders: gout, porphyria
• Inherited metabolic disorder: Fabry disease^[39]

Persistent fever that cannot be explained after repeated routine clinical inquiries is called fever of unknown origin.

Teething is not a cause.^[40]