MODEL RELEASED. Measuring oral temperature. Child clutching her forehead while her mother checks her temperature using a digital thermometer. The metal bulb of the thermometer (left) is placed in the mouth, and the temperature is displayed in the window. In this case the temperature displayed is 39.3 degrees Celsius, which is a high fever (pyrexia). This is defined as a temperature above 37 degrees Celsius, when measured orally. Most fevers are caused by bacterial or viral infections and may be accompanied by other symptoms such as shivering, sweating, thirst and hot skin.