Fever is from the heat of the Hell-Fire so cool it down with water
حَدَّثَنَا أَبُو سَلَمَةَ، يَحْيَى بْنُ خَلَفٍ حَدَّثَنَا عَبْدُ الأَعْلَى، عَنْ سَعِيدٍ، عَنْ قَتَادَةَ، عَنِ الْحَسَنِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ قَالَ " الْحُمَّى كِيرٌ مِنْ كِيرِ جَهَنَّمَ فَنَحُّوهَا عَنْكُمْ بِالْمَاءِ الْبَارِدِ " .
It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said: “Fever is one of the bellows of Hell, so avert it from yourselves with cold water.”