Normal body temperature varies during the day. It is generally lowest upon awakening in the morning. Food, extra clothing, excitement, and anxiety can all raise the body temperature.

Strenuous exercise can also temporarily raise body temperature to as high as 103 degrees Fahrenheit. Extreme exercise, such as running a marathon, can raise body temperature as high as 107 degrees Fahrenheit, which can be dangerous (if sustained).

A woman's menstrual cycle can also elevate temperature by one degree or more. Certain medications also cause an increase in body temperature.

Most children have higher body temperatures than adults and seem to have more daily variation. Many infants and children develop high fevers, even with minor viral illnesses. A high fever does not directly cause brain damage. However, febrile seizures can occur in some children.

Fevers are described as low grade (102 degrees Fahrenheit or lower) or high grade (above 103 degrees Fahrenheit). They are also described as either spiking (the fever suddenly jumps high, then drops) or cyclic (the fever increases and decreases in a regular manner).

Bacteria, viruses, or their toxins may be released into the bloodstream and can cause chills and shivering in the patient with a fever. Chills can also occur during a temperature spike within a fever.

Heatstroke can result from too much exercise without water or enough salt, and temperature can rise to 106 degrees Fahrenheit, which can be dangerous (if sustained).

Unexplained fevers that continue for days or weeks are referred to by doctors as fevers of undetermined origin (FUO). Most are eventually found to be caused by a hidden infection.

• Viral and bacterial infections
• Colds or flu-like illnesses
• Sore throats and strep throat
• Earaches (otitis media)
• Viral gastroenteritis or bacterial gastroenteritis
• Acute bronchitis
• Infectious mononucleosis
• Urinary tract infections
• Upper respiratory infections (such as tonsillitis, pharyngitis or laryngitis)
• Medications (such as antibiotics, antihistamines, barbiturates, and drugs for high blood pressure)
• Occasionally, more serious problems like pneumonia, appendicitis, tuberculosis(TB), and meningitis
• Fever can occur in infants who are overdressed in hot weather or a hot environment
• Collagen vascular disease, rheumatoid diseases, and autoimmune disorders
• Juvenile rheumatoid arthritis
• Lupus erythematosus
• Periarteritis nodosa
• AIDS and acute HIV infection
• Inflammatory bowel disease
• Regional enteritis
• Ulcerative colitis
• Cancer
• Leukemia
• Neuroblastoma
• Hodgkin's disease
• Non-Hodgkin's lymphoma