fever fever 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. 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. 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. rarely a fever may trigger a febrile seizure, with this being more common in young children. fevers do not typically go higher than 41 to 42 °c (106 to 108 °f). feverother namespyrexia, febrile response, febrile"the sick girl", 1882, statens museum for kunstspecialtyinfectious disease, pediatricssymptomsinitially: shivering, feeling cold, chillslater: flushed, sweatingcomplicationsfebrile seizurecausesvirus, bacteria, increase in the body's temperature set pointdiagnostic methodtemperature > between 37.2 and 38.3 °c (99.0 and 100.9 °f)differential diagnosishyperthermiatreatmentbased on underlying cause, not required for fever itselfmedicationibuprofen, paracetamol (acetaminophen)frequencycommon 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, covid-19, and malaria. non-infectious causes include vasculitis, deep vein thrombosis, connective tissue disease, side effects of medication or vaccination, and cancer. 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. treatment to reduce fever is generally not required. treatment of associated pain and inflammation, however, may be useful and help a person rest. medications such as ibuprofen or paracetamol (acetaminophen) 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. hyperthermia requires treatment. fever is 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. while fever evolved as a defense mechanism, treating a fever does not appear to improve or worsen outcomes. fever is often viewed with greater concern by parents and healthcare professionals than is usually deserved, a phenomenon known as fever phobia. associated symptoms 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. diagnosis temperature classification core (rectal, esophageal, etc.) hypothermia <35.0 °c (95.0 °f) normal 36.5–37.5 °c (97.7–99.5 °f) fever >37.5 or 38.3 °c (99.5 or 100.9 °f) hyperthermia >37.5 or 38.3 °c (99.5 or 100.9 °f) hyperpyrexia >40.0 or 41.0 °c (104.0 or 105.8 °f) note: the difference between fever and hyperthermia is the underlying mechanism. different sources have different cut-offs for fever, hyperthermia and hyperpyrexia. a 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) an ear (tympanic) or forehead (temporal) temperature may also be used. temperature in the mouth (oral) is at or over 37.2 °c (99.0 °f) in the morning or over 37.7 °c (99.9 °f) in the afternoon temperature under the arm (axillary) is usually about 0.6 °c (1.1 °f) below core body temperature. in adults, the normal range of oral temperatures in healthy individuals is 35.7–37.7 °c (96.3–99.9 °f) among men and 33.2–38.1 °c (91.8–100.6 °f) among women, while when taken rectally it is 36.7–37.5 °c (98.1–99.5 °f) among men and 36.8–37.1 °c (98.2–98.8 °f) among women, and for ear measurement it is 35.5–37.5 °c (95.9–99.5 °f) among men and 35.7–37.5 °c (96.3–99.5 °f) among women. normal body temperatures vary depending on many factors, including age, sex, time of day, ambient temperature, activity level, and more. normal daily temperature variation has been described as 0.5 °c (0.9 °f).: 4012 a raised temperature is not always a fever. for example, the temperature rises in healthy people when they exercise, 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. hyperthermia hyperthermia is an elevation of body temperature over the temperature set point, due to either too much heat production or not enough heat loss. hyperthermia is thus not considered fever.: 103 hyperthermia should not be confused with hyperpyrexia (which is a very high fever).: 102 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.: 103 types different fever patterns observed in plasmodium infectionsvarious 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, 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. in addition, there is disagreement regarding whether a specific fever pattern is associated with hodgkin's lymphoma—the pel–ebstein fever, with patients argued to present high temperature for one week, followed by low for the next week, and so on, where the generality of this pattern is debated. persistent fever that cannot be explained after repeated routine clinical inquiries is called fever of unknown origin. 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. 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 or 41 °c (104 or 106 °f); the range of hyperpyrexias includes cases considered severe (≥ 40 °c) and extreme (≥ 42 °c). it differs from hyperthermia in that one's thermoregulatory system's set point for body temperature is set above normal, then heat is generated to achieve it. in contrast, hyperthermia involves body temperature rising above its set point due to outside factors. 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. a common cause of hyperpyrexia is an intracranial hemorrhage. other causes in emergency room settings include sepsis, kawasaki syndrome, neuroleptic malignant syndrome, drug overdose, serotonin syndrome, and thyroid storm. differential diagnosis fever is a common symptom of many medical conditions: infectious disease, e.g., covid-19, dengue, ebola, gastroenteritis, hiv, influenza, lyme disease, rocky mountain spotted fever, secondary syphilis, malaria, mononucleosis, as well as infections of the skin, e.g., abscesses and boils. immunological diseases, e.g., relapsing polychondritis, autoimmune hepatitis, granulomatosis with polyangiitis, horton disease, inflammatory bowel diseases, kawasaki disease, lupus erythematosus, sarcoidosis, and still's disease; tissue destruction, as a result of cerebral bleeding, crush syndrome, hemolysis, infarction, rhabdomyolysis, surgery, etc.; cancers, particularly blood cancers such as leukemia and lymphomas; metabolic disorders, e.g., gout, and porphyria; and inherited metabolic disorder, e.g., 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. teething is not a cause of fever. function hyperthermia: characterized on the left. normal body temperature (thermoregulatory set point) is shown in green, while the hyperthermic temperature is shown in red. as can be seen, hyperthermia can be conceptualized as an increase above the thermoregulatory set point.hypothermia: characterized in the center: normal body temperature is shown in green, while the hypothermic temperature is shown in blue. as can be seen, hypothermia can be conceptualized as a decrease below the thermoregulatory set point.fever: characterized on the right: normal body temperature is shown in green. it reads "new normal" because the thermoregulatory set point has risen. this has caused what was the normal body temperature (in blue) to be considered hypothermic. immune function fever is thought to contribute to host defense, as the reproduction of pathogens with strict temperature requirements can be hindered, and the rates of some important immunological reactions are increased by temperature. fever has been described in teaching texts as assisting the healing process in various ways, including: increased mobility of leukocytes;: 1044 enhanced leukocyte phagocytosis;: 1030 decreased endotoxin effects;: 1029 and increased proliferation of t cells.: 1030 : 212 advantages and disadvantages having a fever response in response to an infectious disease is generally regarded as protective, whereas fever in non-infections may be maladaptive. studies have not been consistent on whether treating fever generally worsens or improves mortality risk. benefits or harms may depend on the type of infection, health status of the patient and other factors. studies using warm-blooded vertebrates suggest that they recover more rapidly from infections or critical illness due to fever. in sepsis, fever is associated with reduced mortality. pathophysiology of fever induction