Your normal body temperature varies throughout the day — it's lower in the morning and higher in the late afternoon and evening. In fact, your normal temperature can range from about 97 F (36.1 C) to 99 F (37.2 C). Although most people consider 98.6 F (37 C) normal, your temperature may vary by a degree or more. Other factors, such as your menstrual cycle or heavy exercise, can affect your temperature.

A fever might be caused by:

  • A virus
  • A bacterial infection
  • Heat exhaustion
  • Extreme sunburn
  • Certain inflammatory conditions such as rheumatoid arthritis — inflammation of the lining of your joints (synovium)
  • A malignant tumor
  • Some medications, such as antibiotics and drugs used to treat high blood pressure or seizures
  • Some immunizations, such as the diphtheria, tetanus and acellular pertussis (DTaP) or pneumococcal vaccines

Sometimes it's not possible to identify the cause of a fever. If you have a temperature of 101 F (38.3 C) or higher for more than three weeks and your doctor isn't able to find the cause after extensive evaluation, the diagnosis may be fever of unknown origin.

Jun. 01, 2011