Fever in adults: When to worry

The average body temperature is 98.6° F (37°C). But "normal" body temperature varies from person to person. It also changes during the day, rising a bit after you eat or exercise. Body temperature is often higher in the afternoon than it is when you wake up in the morning. Fever means a body temperature of 100.4° F (38°C) or higher. An infection is the most common cause of fever. Examples include the flu, pneumonia, food poisoning, and bladder infection.

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