A fever is usually when your body temperature is 38C (100.4F) or more. You may feel warm, cold or shivery. You can find out if you have a fever by using a thermometer to take your temperature. What causes a fever? A fever is your body's natural response to many common illnesses such as: flu; tonsillitis; kidney or urinary tract infections (UTIs). The average normal body temperature for an adult is 98.2°F (37°C) 1. Most medical professionals consider a temperature above 100.4°F (38°C) as a sign of a fever 2.