A fever is a temporary rise in body temperature, usually caused by an infection. Body temperatures vary slightly from person to person and at different times of day, but the average temperature has traditionally been defined as 98.6 F (37 C) . A temperature taken using a mouth thermometer (oral temperature) thats 100 F (37.8 C) or higher is generally considered to be a fever. However, there are no strict guidelines for what temperature is a fever, because body temperature varies by person, time of day, and method of measurement. Healthcare providers most often say a fever temperature is 100.4 degrees F (38 degrees C) when measured orally (by mouth) . In adults, fevers less than 103 degrees F (39.4 degrees C) typically aren’t dangerous and aren’t a cause for concern. However, if a fever rises above that level, it is recommended to call a healthcare provider for treatment. If a fever is accompanied by severe symptoms such as seizures, loss of consciousness, confusion, stiff neck, trouble breathing, severe pain anywhere in the body, or swelling or inflammation of any part of the body, it may be a sign of a serious or life-threatening illness, and immediate medical attention should be sought.