Fever remains the most common concern for which parents bring their child to the emergency department. Fever has traditionally been defined as a rectal temperature over 100.4 F or 38.0 C. Temperatures measured at other sites are usually lower. The threshold for defining a fever does vary significantly between different individuals, since body temperatures can vary by as much as 1 F.
Fever itself is not life-threatening unless it is extremely and persistently high, such as greater than 107 F (41.6 C) when measured rectally. Fever may indicate the presence of a serious illness, but usually a fever is caused by common infections which are not serious. The part of the brain called the hypothalamus controls body temperature. The hypothalamus increases the body's temperature as a way to fight the infection. However, many conditions other than infections may cause a fever.