Fever is when the body's temperature is raised above its normal level. Typically, it's slightly lower in the morning and slightly higher in the evening. Like most warm-blooded mammals, humans seem to operate best with a relatively constant internal temperature. The brain's hypothalamus acts somewhat like a thermostat to regulate the body's temperature. This higher setting is usually due to the presence of small molecules, known as pyrogens, in the blood.
Pyrogens can come from external substances such as bacteria, viruses, or toxins.
Sometimes tumors and other substances created within the body can release pyrogens. Under some circumstances, the body's elevated temperature can help fight off an infection, but this isn't always the case. Excessive fever can cause problems such as dehydration and discomfort. Acetaminophen and other fever-reducing medications work by decreasing the levels of pyrogens at the hypothalamus. Some doctors may have different suggestions for when to call.