Pyrexia is any temperature over 38.2 (100.7F) but there are two types of pyrexia. Intrinsic is induced by your immune system - typically white blood cells called Macrophages that produce pyrogens (heat stimulating chemicals ) instructing your brains thermostat to raise. These intrinsic pyrexia are simply called FEVER and are most commonly triggered by infection. Extrinsic Pyrexia is called Hyperthermia and triggered by an outside influence like heat, or certain medications (Serotonergic, Dopamin