Fever is a temporary increase in body temperature, often due to an underlying infection or illness. It is a common symptom that indicates the body's immune system is fighting off invaders like bacteria, viruses, or other pathogens. A normal body temperature is typically around 98.6°F (37°C), but a fever is usually defined as a temperature above 100.4°F (38°C). Causes of Fever: Fever can be cause..