As you aware, if temperature increases (Absence of fever)after 31 degree Celsius , Warm sensitive neurons increase their firing rate and inhibit Cold sensitive neurons as core temperature increases. As temperature drops, the firing rate of Warm sensitive neurons decreases, reducing their inhibition, and Cold sensitive neurons which respond by increasing their firing rates. On the contrary to increase of temperature, in fever the firing rate of Warm sensitive neurons decreases, the firing rate of Cold sensitive neurons increases as core temperature increases. inhibit warm sensitive neurons. The temperature increasing and decreasing controlled by the brain. The firing rate of Warm sensitive neurons and Cold sensitive neurons also controlled by the brain. When the disease becomes threat to life or organs, blood circulation decreases. Temperature of fever will emerges to increase prevailing essential blood circulation. WBC and their products stimulate the brain to increase temperature by increasing the firing rate of Cold sensitive neurons and decreasing the firing rate of Warm sensitive neurons. And it acts as a protective covering of the body to sustain life. There is no way other than this for a sensible and discreet brain to increase temperature.