Thermometer measuring fever. Gloved hand holds a thermometer in degrees Fahrenheit (F) and Celsius (C), showing fever. It registers a temperature of 101 F (38.3 C). A rise in body temperature to above 37 C measured in the mouth or 37.7 C in the rectum is known as fever. Most fevers are due to bacterial or viral infection. Symptoms produced may include shivering, headache, sweating, and a fast heartbeat. In extreme cases confusion, delirium and even coma may result. The thermometer consists of a sealed narrow-bore glass tube with a bulb at one end. Mercury inside the tube expands when heated and extends along the scale. This thermometer measures temperatures of 94-108 F.