Fever, also known as pyrexia, is a medical symptom which describes an increase in temperature to levels which are above normal (37 degrees Celsius, 98.6 degrees Fahrenheit). An adaptive mechanism, fever is the body's reaction to pathogens; it attempts to kill them by overheating them. Most fevers are caused by infections and almost all infectious diseases cause fever. When a patient has or is suspected of having a fever, that person's body temperature is measured using a thermometer.
Substances which induce fever are called pyrogens. When pyrogens are secreted directly by external pathogens, they are exogenous. Fever usually occurs in response to endogenous pyrogens (the most prominent one of which is interleukin 1) which are released from inflammatory cells, such as macrophages. These pyrogens act on the hypothalamus (the body's thermostat), resetting it to a higher temperature and in doing so invoking the body's temperature raising mechanisms and raising the body temperature to a level above normal. The body has a number of techniques to raise its temperature.
* Shivering, which involves physical movement that produces heat.
* Vasoconstriction, which entails the constriction of blood flow beneath the skin and thus reduces the amount of heat lost from the body.
The body's temperature is maintained at that level until the effects of the pyrogens wear off.
Pyrexia can be classed as low grade (38 to 39 °C), moderate (39 to 40 °C), or high grade (more than 40 °C) depending on how much the body temperature has deviated from normal.
Fever may be of benefit and is part of the body's response to a disease; however, if the fever goes above 42 °C then it may cause significant tissue damage and would most certainly be harmful. This is termed hyperpyrexia.
Temperature normally fluctuates over the day, and the same applies to fever. If this characteristic pattern is lost, the raised body temperature may be due to hyperthermia, a more dangerous disorder. Hyperthermia is commonly caused by overheating or sunstroke.
Drugs that reduce fever are known as antipyretics. Common antipyretics are acetaminophen, also called paracetamol, and NSAIDs such as ibuprofen. A popular household remedy is soaking a cloth in cold water and placing it on the patient's forehead.
Sometimes, for health reasons, mild fevers are intentionally induced. Cancer cells, for example, are known to die at lower temperatures than normal body cells, and therefore can sometimes be fought with fevers.
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