MODEL RELEASED. Measuring oral temperature. General practice doctor takes the temperature of a feverish young girl with a thermometer. The girl is resting against her mother. After a few minutes in the girl's mouth the thermometer will register the correct temperature. Fever, also known as pyrexia, is defined as a body temperature above 37 degrees celsius if measured in the mouth. Most fevers are caused by bacterial or viral infections and may be accompanied by other symptoms such as shivering, sweating, thirst and hot skin.