MODEL RELEASED. Feverish child. Four-year-old girl with a fever having her temperature checked by her mother. The mother has placed her hand on her daughter's forehead to feel how hot is it. Fever, known as pyrexia, is defined as a body temperature above 37 Celsius (98.6 Fahrenheit), when the temperature is taken orally. Most fevers are caused by bacterial or viral infections and may be accompanied by other symptoms such as shivering, sweating, headaches, thirst and hot skin.