What is low-grade fever? A fever is an increase in the body temperature above normal. A low-grade fever is a mild elevation of the temperature above normal. Your temperature measurements fluctuate through the day and vary depending upon the site of measurement. Generally, a child is considered to have a fever if the temperature is at or above 100.4 degrees Fahrenheit rectally, 99.5 degrees Fahrenheit orally, or 99 degrees Fahrenheit in the armpit. In an adult, a fever is generally defined as 100 degrees Fahrenheit (37.8 degrees Celsius) or greater