Fever is caused by germs, namely bacteria. However, Muhammad believed that it is a spark from hell to punish humans.
Abdullah bin ‘Umar said, “The Prophet said, ‘Fever is from the heat of Hell, so put it out (cool it) with water.’ ” Nafi’ added: ‘Abdullah used to say, “O Allah! Relieve us from the punishment,” (when he suffered from fever).
Sahih al-Bukhari 5723
Vol. 7, Book 71, Hadith 619
Narrated Abu Jamra Ad-Dabi:
I used to sit with Ibn `Abbas in Mecca. Once I had a fever and he said (to me), “Cool your fever with Zamzam water, for Allah’s Messenger said: ‘It, (the Fever) is from the heat of the (Hell) Fire; so, cool it with water (or Zamzam water).
Sahih al-Bukhari 3261
Vol. 4, Book 54, Hadith 483