A fever is an abnormally high human body temperature. For most people, the normal body temperature is 98.6 degrees Fahrenheit, and that fluctuates during the day. Lower temperatures usually happen in the early morning and higher temperatures usually occur in the afternoon.
Viruses, the most common cause of fevers, often travel through a person's bloodstream and are fought by the white blood cells. Bacteria can live and reproduce at 98.6, but can't always survive in higher temperatures. When confronted by harmful bacteria, the white blood cells produce pyrogens. The pyrogens travel to our hypothalamus, the part of the brain that regulates body temperature. The hypothalamus raises our body temperature, causing the harmful bacteria to die or stop reproducing.
Environmental factors can affect the temperature of our body and put us at a higher risk for getting sick. For instance, if it's freezing outside, you might become cold and your temperature might drop. At this temperature, your body can't fight off the viruses that are always around and usually fought off.