Credit to Bassam Zawadi There are Non-Muslims who bring up the following hadeeth: Saheeh Bukhari Volume 4, Book 54, Number 484: Narrated Rafi bin Khadij: I heard the Prophet saying, "Fever is from the heat of the (Hell) Fire; so cool it with water." —— And they claim that this is a scientific error, since science today shows that fevers come from something else and not hellfire. These kinds of arguments should be expected from atheists and not Christians who believe in the unseen realm. However, since many Christian missionaries employ double standards, they do bring up this argument. :Ibn Hajar Al Asqalani in his commentary said قال ابن حجر: (من فيح أو فوج جهنم بمعنى سطوع حرها ووهجه، واختلف في نسبتها إلى جهنم فقيل حقيقة، واللهب الحاصل في جسم المحموم قطعة من جهنم وقد قدر الله ظهورها بأسباب تقتضيها ليعتبر العباد بذلك، كما أن أنواع اللذة والفرح من نعيم الجنة أظهرها في هذه الدار عبرة ودلالة. وقيل بل الخبر مورد التشبيه والمعنى أن حر الحمى شبيه بحر جهنم تنبيهاً للنفوس على شدة حر النار وأن هذه الحرارة الشديدة شبيهة بفيحها وهو ما يصيب من قرب منها من حرها). ". from the 'Faih' or the 'Fauj' of Hell, which means, 'from its radiation and heat.' There is a difference of opinion regarding its relation to Hell. It was said that the relation is literal. Therefore, the heat felt by one who suffers from a fever is a part of Hell, which Allah made to appear through means that He has measured so that the slaves heed to this lesson. Similarly, types of delight and elation are parts of Paradise's delight and Allah made these to appear in this life as a lesson and a reminder. It was also said that this Hadeeth (Khabar) is a figure of speech. Thus, in this case, it would mean that the heat of fever is similar to the heat of Hell to alert one's inner self to the intensity of the heat of the Fire and that the heat of fever is similar to the Fire's Faih' in that it touches those near it." (Fathul Baari) The Muslim response basically is "this is a matter of the unseen, which is beyond scientific analysis, and we don't know how Allah allows Hellfire to cause fevers for people on Earth, but we do know that He is capable of doing so."