At what body temperature does it become dangerous?

100.4 F: A fever is defined as temperature above 100.4 f. Generally, temperatures above 104 is considered hyperpyrexia, where some of your body's cells and proteins will be destroyed. This is very dangerous and means there is a serious pathology taking place. If you notice high fevers, temperature rising quickly or any major temperature swings, go to the er or call your physician immediately.