Fever, also referred to as pyrexia, is defined as having a temperature above the normal range due to an increase in the body's temperature set point. There is not a single agreed-upon upper limit for normal temperature with sources using values between 37.2 and 38.3 °C (99.0 and 100.9 °F) in humans.

How does pyrexia occur?

The pyrogens inhibit heat-sensing neurons and excite cold-sensing ones, and the altering of these temperature sensors deceives the hypothalamus into thinking the body is cooler than it actually is. In response, the hypothalamus raises the body's temperature above the normal range, thereby causing a fever.

At what point does Hyperpyrexia occur?

When your body temperature exceeds 106°F (41.1°C) due to a fever, you're considered to have hyperpyrexia.