When you have a fever, it’s because your hypothalamus, which is sometimes called the “thermostat” of the body, has elevated your body temperature.v This can occur due to virus, bacterial infection, heat exhaustion, inflammatory conditions like rheumatoid arthritis, a malignant tumor, and certain medications.vi It’s normal for your body temperature to shift slightly throughout the day, being a little lower in the morning and higher in the afternoon and evening. Anything in the range between 97° F (36.1° C) and 99 F° (37.2° C) is still considered a normal body temperature.vii When your body temperature gets over 100.4° F (38°C), you have entered fever territory.