In adults, doctors consider any temperature above 100.4 degrees Fahrenheit a high fever, MedicineNet states. Doctors recommend seeking medical attention immediately when a fever reaches 104 degrees.
The average body temperature of a healthy adult is 98.6 degrees Fahrenheit, with a minor variation of 1 degree or less, depending on the individual, explains MedicineNet. The immune system often triggers a fever to increase body temperature and fight off infectious agents, such as viruses and toxins, that can't thrive in warmer conditions; however, at temperatures above 100.4 degrees Fahrenheit, adults may develop symptoms such as headaches, chills, heart palpitations, dizziness and muscle aches. A fever at or above 104 degrees produces more severe reactions, including hallucinations and convulsions.