What causes fever?
Most fevers last only a few hours or days and are not dangerous; however, they may cause a great deal of discomfort. A rectal temperature of greater than 101.8 F (38.8 C), an oral temperature of more than 100 F (37.8 C), or an armpit temperature of greater than 99 F (37.2 C) is considered significantly abnormal. Fevers are usually due to viral or bacterial infections; however, they also can be due to cancers, injury to tissue (for example, heart attacks), hyperthyroidism, other illnesses in which there is inflammation, and dehydration. Additionally, many different drugs have been reported to cause "drug fever."
Harmful effects of fever (for example, dehydration, changes in consciousness, seizures, or coma) are likely to occur at temperatures above 106 F. Lower fevers can be dangerous in persons with heart disease, since fever increases the effort required by the heart to pump blood.
Two percent to four percent of children between the ages of 6 months and 5 years (usually before age 3) with high fevers will experience febrile seizures; though these seizures generally last no more than 15 minutes. Moreover, children who experience febrile seizures have a higher risk of developing epilepsy later in life.
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