A fever is any body temperature elevation over 100°F (37.8°C). A healthy person's body temperature fluctuates between 97°F (36.1°C) and 100°F (37.8°C), with the average being 98.6°F (37°C). The bodymaintains stability within this range by balancing the heat produced by the metabolism with the heat lost to the environment. The "thermostat" that controls this process is located in the hypothalamus, a small structure deep withinthe brain. The nervous system constantly relays information about the body'stemperature to the thermostat, which activates different physical responsesto cool or warm the body. These responses include: decreasing or increasing the flow of blood from the body's core, where it is warmed, to the surface, where it is cooled; slowing down or speeding up the rate at which the body turns food into energy (metabolic rate); inducing shivering, which generates heatthrough muscle contraction; and inducing sweating, which cools the body through evaporation. A fever occurs when the thermostat resets at a higher temperature, primarily in response to an infection. To reach the higher temperature, the body moves blood to the warmer interior, increases the metabolic rate,and induces shivering. The "chills" that often accompany a fever are caused by the movement of blood to the body's core, leaving the surface and extremities cold. Once the higher temperature is achieved, the shivering and chills stop. When the infection is overcome or drugs such as aspirin or acetaminophen (Tylenol) are taken, the thermostat resets to normal and the body's cooling mechanisms switch on: the blood moves to the surface and sweating occurs. Fever is an important part of the immune response. Physicians believe that an elevated body temperature increases the production of cells that fight off bacteria or viruses; inhibits the growth of some bacteria, while speeding up the chemical reactions that help the body's cells repair themselves;and speeds the arrival of white blood cells to the sites of infection.
Fevers are primarily caused by viral or bacterial infections, such as pneumonia or influenza. Other conditions that can cause a fever include: allergic reactions; autoimmune diseases; trauma, such as breaking a bone;cancer; excessive exposure to the sun; intense exercise; hormonal imbalances; certain drugs; and damage to the hypothalamus. How long a fever lasts and how high it may go depends on its cause, the age of the patient,and his or her overall health. Most fevers caused by infections appear suddenly and then go away as the immune system defeats the infectious agent.An infectious fever may also rise and fall throughout the day, reaching itspeak in the late afternoon or early evening. A low-grade fever that lasts forseveral weeks is associated with autoimmune diseases such as lupus or with some cancers, particularly leukemia and lymphoma.
A fever is usually diagnosed using a thermometer. Determining the cause of the fever is important. The presence or absence of accompanying symptoms, a patient's medical history, and information about what he or she may have ingested, any recent trips taken, or possible exposures to illness help the physician make a diagnosis. Blood tests can help identify an infectious agent by detecting the presence of antibodies against it or providing samples for growth of the organism in a culture. Blood tests can also provide the doctor with white blood cell counts. Ultrasound tests, magnetic resonance imaging (MRI) tests, or computed tomography (CT) scans may be ordered if the doctor cannot determine the cause of a fever. The most effective treatment for a fever is to address its cause, such as through the administration of antibiotics. Also, because a fever helps the immune system fight infection, it usually should be allowed to run its course. Drugs to lower fever such as aspirin, acetaminophen (Tylenol), and ibuprofin (Advil) can be used if apatient (particularly a child) is uncomfortable. Do not give aspirin to a child or adolescent; it has been linked to an increased risk of Reye's syndrome. Bathing a patient in cool water can also help alleviate a high fever. A fever requires emergency treatment: in a newborn (three months or younger) with a fever over 100.5°F (38°C), in an infant or child with a fever over 103°F (39.4°C), or if it's accompanied by severe headache, neck stiffness, mental confusion, or severe swelling of the throat. A very high fever in a small child can trigger seizures and should be treated immediately. A fever accompanied by the above symptoms can indicate the presence of a serious infection, such as meningitis, and should be brought to the immediate attention of a physician. Most fevers caused by infectionend as soon as the immune system rids the body of the pathogen and do not produce any lasting effects. The prognosis for fevers associated with more chronic conditions, such as autoimmune disease, depends upon the overall outcomeof the disorder.