question: Why does our body temperature increase when we are sick?
comment: The bacteria or viruses that are making us ill thrive best at normal body temperature. As part of a defense mechanism, our body will therefore increase its temperature when ill in attempts to make it less hospitable to them, giving our immune system a better chance at containing and eradicating the contagion.
passages: With fever, the body's core temperature rises to a higher temperature through the action of the part of the brain that controls the body temperature; with hyperthermia, the body temperature is raised without the influence of the heat control centers.
  
  Section::::Concepts.:Hypothermia.
  
  In hypothermia, body temperature drops below that required for normal metabolism and bodily functions. In humans, this is usually due to excessive exposure to cold air or water, but it can be deliberately induced as a medical treatment. Symptoms usually appear when the body's core temperature drops by below normal temperature.
  
  Section::::Concepts.:Basal body temperature.
   Fever, also known as pyrexia and febrile response, 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 . The increase in set point triggers increased muscle contractions and causes a feeling of cold. 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. This is more common in young children. Fevers do not typically go higher than .
   A fever occurs when the core temperature is set higher, through the action of the pre-optic region of the anterior hypothalamus. For example, in response to a bacterial or viral infection, certain white blood cells within the blood will release pyrogens which have a direct effect on the anterior hypothalamus, causing body temperature to rise, much like raising the temperature setting on a thermostat.
  
  In contrast, hyperthermia occurs when the body temperature rises without a change in the heat control centers.
   Section::::Thermodynamics.
  
  The ambient temperature of the body directly affects blood flow through microvasculature. Changes in temperature affect the viscosity of blood and the surface tension. Surface tension decreases with increasing temperature, decreasing the minimum flow rate (see Surface Tension). The decrease in minimum flow rate by higher temperatures allows more blood to flow and dissipate heat throughout the body. Temperature greatly affects blood flow by influencing the diameter of flow. Decreases and increases in temperature trigger vasoconstriction and vasodilation respectively.
  
  Section::::Thermodynamics.:Vasoconstriction.
   In many respects, the hypothalamus works like a thermostat. 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, then shivering begins in order to use muscle movements to produce more heat. When the hypothalamic set point moves back to baseline either spontaneously or with medication, the reverse of these processes (vasodilation, end of shivering and nonshivering heat production) and sweating are used to cool the body to the new, lower setting.
   Blood circulation transports heat throughout the body, and adjustments to this flow are an important part of thermoregulation. Increasing blood flow to the surface (e.g., during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss. In contrast, when the external temperature is low, blood flow to the extremities and surface of the skin is reduced and to prevent heat loss and is circulated to the important organs of the body, preferentially.
  
  Section::::Physiology.:Rate of blood flow.
   Temperature can be recorded in order to establish a baseline for the individual's normal body temperature for the site and measuring conditions. The main reason for checking body temperature is to solicit any signs of systemic infection or inflammation in the presence of a fever (temp  38.5 °C/101.3 °F or sustained temp  38 °C/100.4 °F), or elevated significantly above the individual's normal temperature. Other causes of elevated temperature include hyperthermia.
   A temperature "setpoint" is the level at which the body attempts to maintain its temperature. When the setpoint is raised, the result is a fever. Most fevers are caused by infectious disease and can be lowered, if desired, with antipyretic medications.
  
  An early morning temperature higher than or a late afternoon temperature higher than is normally considered a fever, assuming that the temperature is elevated due to a change in the hypothalamus's setpoint. Lower thresholds are sometimes appropriate for elderly people. The normal daily temperature variation is typically , but can be greater among people recovering from a fever.
   An organism at optimum temperature is considered "afebrile" or "apyrexic", meaning "without fever". If temperature is raised, but the setpoint is not raised, then the result is hyperthermia.
  
  Section::::Concepts.:Hyperthermia.
   Fever can also be behaviorally induced by invertebrates that do not have immune-system based fever. 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". Honeybee colonies are also able to induce a fever in response to a fungal parasite "Ascosphaera apis".
  
  Section::::Further reading.
  
  BULLET::::- Rhoades, R. and Pflanzer, R. Human physiology, third edition, chapter 27 "Regulation of body temperature", p. 820 "Clinical focus: pathogenesis of fever".
  
  Section::::External links.
  
  BULLET::::- Fever and Taking Your Child's Temperature
   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:
  
  BULLET::::- Temperature in the anus (rectum/rectal) is at or over
  
  BULLET::::- Temperature in the mouth (oral) is at or over
  
  BULLET::::- Temperature under the arm (axillary) or in the ear (tympanic) is at or over
   The neural activation mechanisms involved in the regulation of body temperature are largely undefined. It is known that sympathetic pathways are involved in increasing heat production and reducing heat loss and are activated by neurons in the rostal medullary raphe (RMR). These neurons were identified as playing an important role in the elevation of body temperature during both cold exposure and induced fever by observation that hyperpolarization prior to exposure to these conditions inhibits the elevation of body temperature in response.
  
  Section::::Role in thermoregulation.:Febrile response.
   BULLET::::- Arteriolar vasodilation occurs. The smooth muscle walls of the arterioles relax allowing increased blood flow through the artery. This redirects blood into the superficial capillaries in the skin increasing heat loss by convection and conduction.
  
  Section::::In hot and humid conditions.
   Infections are the most common cause of fevers, but as the temperature rises other causes become more common. Infections commonly associated with hyperpyrexia include roseola, measles and enteroviral infections. Immediate aggressive cooling to less than has been found to improve survival. 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.
  
  Section::::Diagnosis.:Hyperthermia.
   In healthy adults, the range of normal, healthy temperatures for oral temperature is , for rectal it is , for tympanic membrane (the ear drum) it is , and for axillary (the armpit) it is . 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).
   Section::::Pathophysiology.:Hypothalamus.
  
  The brain ultimately orchestrates heat effector mechanisms via the autonomic nervous system or primary motor center for shivering. These may be:
  
  BULLET::::- Increased heat production by increased muscle tone, shivering and hormones like epinephrine (adrenaline)
  
  BULLET::::- Prevention of heat loss, such as vasoconstriction.
  
  In infants, the autonomic nervous system may also activate brown adipose tissue to produce heat (non-exercise-associated thermogenesis, also known as non-shivering thermogenesis). Increased heart rate and vasoconstriction contribute to increased blood pressure in fever.
  
  Section::::Pathophysiology.:Usefulness.
   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 may represent a clinically significant fever.
  
  Section::::Diagnosis.:Types.
   Temperature also varies with the change of seasons during each year. This pattern is called a "circannual" rhythm. Studies of seasonal variations have produced inconsistent results. People living in different climates may have different seasonal patterns.
  
  Increased physical fitness increases the amount of daily variation in temperature.
  
  With increased age, both average body temperature and the amount of daily variability in the body temperature tend to decrease. Elderly patients may have a decreased ability to generate body heat during a fever, so even a somewhat elevated temperature can indicate a serious underlying cause in geriatrics.
  
  Section::::Variations.:Measurement methods.
   Section::::Variation in animals.:Variations due to fever.
  
  Fever is a regulated elevation of the set point of core temperature in the hypothalamus, caused by circulating pyrogens produced by the immune system. To the subject, a rise in core temperature due to fever may result in feeling cold in an environment where people without fever do not.
  
  Section::::Variation in animals.:Variations due to biofeedback.
  
  Some monks are known to practice Tummo, biofeedback meditation techniques, that allow them to raise their body temperatures substantially.
  
  Section::::Low body temperature increases lifespan.
   Growing at host body temperature () is an important requirement for pathogenesis. Some strains of "S. schenckii" are restricted to growing at and consequently usually cause disease only on the skin as it is cooler than the body's interior. Those that are capable of growth at body temperature are more often associated with disseminated disease.
  
  Section::::Immune response.
  
  Infection by "S. schenckii" is generally self-limiting in immunocompetent hosts. The immune response prevents fungal dissemination and is the reason that most "Sporothrix" infections are cutaneous.
  
  Section::::Immune response.:Innate.
   Symptoms of an infection of "P. oryzihabitans" are actually quite vague and similar to the signs that can indicate other illnesses or diseases, so it is relatively difficult to identify when only looking at symptoms. However, in several cases, these infections result after an individual's immune system has been weakened, so it is likely to occur in recovering or ill patients. Most patients, after receiving treatment for another disease or during recovery from surgery, experience chills and increase in body temperature. While these symptoms could mean a variety of things, it is clear that the patient's recovery is halted and that there is an infection of some sort. In an example where a woman developed an infection of "P. oryzihabitans" from a case of sinusitis, she experienced the same chills and elevated temperature, but also nasal discharge containing pus, right facial pain, and a fever.
   One model for the mechanism of fever caused by exogenous pyrogens includes LPS, which is a cell wall component of gram-negative bacteria. An immunological protein called lipopolysaccharide-binding protein (LBP) binds to LPS. The LBP–LPS complex then binds to the CD14 receptor of a nearby macrophage. This binding results in the synthesis and release of various endogenous cytokine factors, such as interleukin 1 (IL-1), interleukin 6 (IL-6), and the tumor necrosis factor-alpha. In other words, exogenous factors cause release of endogenous factors, which, in turn, activate the arachidonic acid pathway. The highly toxic metabolism-boosting supplement 2,4-dinitrophenol induces high body temperature via the inhibition of ATP production by mitochondria, resulting in impairment of cellular respiration. Instead of producing ATP, the energy of the proton gradient is lost as heat.
   Section::::Recording and production.
   Temperature is ultimately regulated in the hypothalamus. A trigger of the fever, called a pyrogen, causes 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.
   Immunosuppresants are often used to battle autoimmune diseases such as lupus. When individuals undergo treatment with immunosuppressants such as glucocorticoids, their bodies's defenses are chemically lowered. As a result, exposure to "C. canimorsus" is more infectious in these individuals than in healthy individuals. Immunosuppressed patients make up about 5% of individuals presenting with "C. canimorsus" symptoms.
  
  Section::::Symptom onset and clinical manifestations.
  