FEVER

Fever is a frequent medical sign that describes an increase in internal body temperature to levels above normal. Technically, any body temperature above the normal oral measurement of 98.6^oF (37^oC) or the normal rectal temperature of 99^oF (37.2^oC) is considered to be elevated. However, these are averages, and your normal temperature may actually be 1^oF (0.6^oC) or more, above or below the average of 98.6^oF. Body temperature can also vary up to 1^oF (0.6^oC) throughout the day.
Thus, fever is not considered medically significant until body temperature is above 100.4^oF (38^oC). Fever serves as one of the body's natural defenses against bacteria and viruses that cannot live at a higher temperature. For that reason, low fevers should normally go untreated, unless accompanied by troubling symptoms.
A feverish individual has a general feeling of cold despite an increased body temperature, and increases in heart rate, muscle tone and shivering, all of which are caused by the body's attempts to counteract the newly-perceived hypothermia and reach the new thermoregulatory set-point
Fevers of 104^oF (40^oC) or higher demand immediate home treatment and subsequent medical attention, as they can result in delirium and convulsions, particularly in children.

Normal human body temperature through oral measurement is 36.8±0.7^oC (98.2±1.3^oF).
Children develop higher temperatures with activities like playing, but this is not fever because their set-point is normal. Elderly patients may have a decreased ability to generate body heat during a fever, so even a low-grade fever can have serious underlying causes in geriatrics.

Different locations in the body from which temperature can be measured:
1.Armpit
2.Mouth, under the tongue
3.Eardrum
4.Anus

Types:
According to one common rule of thumb, fever is generally classified for convenience as an anal (core) temperature of:

The last is a medical emergency, because it approaches the upper limit compatible with human life. If the temperature is taken by another route (mouth, ear, armpit), then the reading needs to be converted to the equivalent core body temperature.

Causes:
Fever is a common symptom of many medical conditions:

• Infectious diseases, e.g. influenza, HIV, malaria, infectious mononucleosis, or gastroenteritis
• Various skin inflammations, e.g. boils, pimples, acne, or abscess
• Immunological diseases, e.g. lupus erythematosus, sarcoidosis, inflammatory bowel diseases
• Tissue destruction, which can occur in hemolysis, surgery, infarction, crush syndrome, rhabdomyolysis, cerebral hemorrhage, etc.
• Drug-induced fevers
• Directly caused by the drug, e.g. lamictal, progesterone, or chemotherapeutics causing tumor necrosis
• As an adverse reaction to drugs, e.g. antibiotics or sulfa drugs.
• After drug discontinuation, e.g. heroin or fentanyl withdrawal
• Reaction to incompatible blood products
• Cancers, most commonly renal cancer and leukemia and lymphomas
• Metabolic disorders, e.g. gout or porphyria
• Thrombo-embolic processes, e.g. pulmonary embolism or deep venous thrombosis
• Persistent fever which cannot be explained after repeated routine clinical inquiries, is called fever of unknown origin.