What is high fever? A high fever is when the body temperature rises above 103 degrees Fahrenheit in an adult (or above 101 degrees Fahrenheit in a child). A fever this high may indicate the presence of a serious infection that has triggered your immune system. The fever is the immune system’s attempt to kill the infection. Very rarely, however, this immune response can result in a fever high enough to cause permanent harm or even life-threatening complications due to overheating. The causes of high fever vary, and they include such relatively routine conditions as sore throat, sinusitis, and urinary tract and other infections. Some causes may be more serious and include such conditions as appendicitis and pneumonia. In rare cases, high fevers can occur without any infection as a result of arthritis, lupus, or certain gastrointestinal and vascular disorders. Treatment for fever may involve treating the underlying cause (usually infection). Other treatments include use of cold compresses to cool your skin, avoiding excess clothing, and medications such as acetaminophen (Tylenol) that can be effective in lowering fever. Seek immediate medical care (call 911) if you, or someone you are with, have a high fever accompanied by other serious symptoms, such as difficulty breathing, severe abdominal pain, confusion, difficulty swallowing, stiff neck, severe headache, or dizziness. Seek prompt medical care if your fever is persistent despite treatment or causes you concern.