In adults, a fever becomes dangerous at 39.4 degrees Celsius, according to Mayo Clinic. For children, it is 40 degrees Celsius, according to Cleveland Clinic.
A dangerously high fever may cause hallucinations, irritability, convulsions, dehydration and mental confusion, says Mayo Clinic. A doctor needs to be called if these symptoms occur. Medical help should also be sought if the patient has a stiff neck, is photosensitive, has chest pain or difficulty breathing, or has a strange skin rash that gets worse over a short period of time. The doctor should also be called if a child vomits repeatedly, suffers severe pains in her head, stomach or other areas, is listless, and has been feverish for a few days.