Fever is from the vehement raging of the fire of Hell, so extinguish it with water. (Using translation from Muslim 2209c)

قَالَ رَسُولُ اللهِ ﷺ «الْحُمَّى مِنْ فَيْحِ جَهَنَّمَ فَأَطْفِئُوهَا بِالْمَاءِ الْبَارِدِ»