MODEL RELEASED. Feverish child. Four-year-old girl recovering in bed from a fever. Her mother is checking her temperature using an oral thermometer. 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.