Temperature regulation occurs in the hypothalamus. Normally, when the body temperature increases the body will respond by causing vasodilation in the periphery. What physiologic change is occurring with fever that allows the body's temperature to increase

The prostaglandins E2  changes the hypothalamic set point of temperature.

Explanation:

The hypothalamic thermoregulatory system keep the body temperature at set point (37.2°C). It detects the change in temperature and respond accordingly to restore the temperature to set point. Such as it constrict and dilate the blood vessels during cold and warmer environment conditions, respectively.

The body immune system cause fever in response to germs such as bacteria and viruses. The prostglandins specifically prostglandin E2 released by immune cells sets the new set point of temperature for hypothalamus i.e. greater than 37.2°C. So the hypothalamus does not detect the change in temperature. The brain act as temperature is normal. That why temperature remains elevated in fever.

The elevated temperature helps the immune system to work more efficiently. when the immune system has finished its job and PGE2 does not effect the brain and temperature will come to normal set point.

thinking it would be physical characteristics