Fever is a temporary rise in body temperature, typically defined as a temperature above 38°C (100.4°F), and is usually a sign that the body is responding to infection or inflammation. It occurs when the immune system releases chemicals that reset the body’s internal thermostat in the brain to help fight viruses, bacteria, or other pathogens. Common accompanying symptoms may include chills, sweating, headache, muscle aches, fatigue, and dehydration. While most fevers are mild and resolve as the underlying illness improves, very high or persistent fevers—especially in young children, older adults, or individuals with weakened immune systems—may require medical evaluation. Treatment generally focuses on addressing the underlying cause, maintaining hydration, rest, and using fever-reducing medications when appropriate.