Model released. Mother comforting her sick, feverish child. Fever, also known as pyrexia, is defined as a body temperature above 37 Celsius (98.6 Fahrenheit) if measured in the mouth, or 37.7 C (99.9 F) if measured in the rectum. Most fevers are caused by bacterial or viral infections; in this case the two year old boy was suffering from chickenpox (varicella). Fever may be accompanied by other symptoms such as shivering, sweating, thirst and hot skin.