A fever occurs when your body temperature rises above the normal range. It’s not an illness itself but rather a symptom of various health conditions. An elevated temperature typically indicates that your immune system is responding to an infection, though it can also be a side effect of certain medications or vaccinations.
Generally, a fever is defined as a body temperature of 100.4°F (38°C) or higher. Different methods of taking a temperature—such as rectal, armpit, ear, forehead, or mouth—can yield slightly different readings, so the threshold for a fever may vary, especially in children.