Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
We ask Allah to grant you the best reward on your interest in understanding Ahadith and bestow upon you steadfastness on the right path. We also highly regard your good impression about this site.
Saheeh al-Bukhari and Muslim both books of Hadith are accepted as Saheeh (authentic) by the whole Ummah and many scholars have reported the consensus of Muslim Ummah on this fact.
A Muslim has to accept and believe firmly in what is proved from the Prophet (Sallallahu Alaihi wa Sallam) and should have strong faith in what is reported from him since it is the revelation of Allah. Allah Says (interpretation of meaning): {Nor does he speak of (his own) desire. It is only an Inspiration that is inspired.}[53:3-4].
One may try to understand the wisdom and reason behind on ruling of Shariah. If he can it is good, otherwise he has to believe in the ruling even if he does not perceive its wisdom. Islamic Shariah does not approve of what is impossible but some minds may not realize the wisdom.
Keeping in mind the above basic rule know that Muslim scholars have explained these two sound Ahadith. They say that the Hadith, There is no ‘Adwa (contagious disease)” means that the natural causes do not work independently rather they work with the permission of Allah and His Divine Decree. Sometimes mixing with a diseased person may cause one to become sick but sometimes one becomes sick without any known reason. Also, according to science a disease spreads through germs and no one has control over a germ. According to medical research not only can an infected person spread disease but a healthy person sometimes spreads germ that causes diseases. A healthy person might come in contact with a germ when meeting a patient or passing through a place, and due to his strong resistance does not get sick. Yet when he meets other healthy persons he spreads the germ to others and they become ill. In this case, Allah made a healthy person to be a reason for spreading the disease.
This research support the fact mentioned in Hadith that Allah alone makes ill whom He wants. We may conclude from the above that everything happens by the permission of Allah. So, believing that some diseases spread themselves is totally false. The clear example for that is that Allah gives children and makes the physical meeting (intercourse) of spouses a means for that. He gives as He wishes, males or females. Sometime they do not get any child though they do what is usually a cause for getting children.
Another evidence for what is mentioned above is that the first person who becomes ill was not infected by any one, but only by the Divine Decree of Allah. It is reported in Saheeh al-Bukhari that the Prophet (Sallallahu Alaihi wa Sallam) said: “There is no 'Adwa (no disease is conveyed from the sick to the healthy without Allah's permission), nor Safar, nor Hama." A bedouin stood up and said, "Then what about my camels? They are like deer on the sand, but when a mangy camel comes and mixes with them, they all get infected with mange." The Prophet said, "Then who conveyed the (mange) disease to the first one?
There is another Hadith that the Prophet (Sallallahu Alaihi wa Sallam) said: Nothing transmits disease to something else.” A Bedouin said: ‘O Allah’s Messenger a camel's lip or tail gets infected with mange and then it infects all camels? The Prophet (Sallallahu Alaihi wa Sallam) said: “Who conveyed the disease to the first camel? Then he said: “No Adwa’, nor Safar; nor Hana; Allah created every soul and has written its life’s span, livelihood and all the calamities that will befall it. [reported by Imam Ahmad, Sheikh al-Albani and al-Arnaout consider it as sound]
Allah also Says (interpretation of meaning): {No calamity befalls, but with the Leave [i.e. decision and Qadar (Divine Preordainments)] of Allâh,….}[64:11]. Another verse reads: {Say: "Nothing shall ever happen to us except what Allâh has ordained for us….}[9:51].
All trials and calamities are from Allah Alone and it is from the Divine Decree. Nevertheless, a person is ordered to avoid all the causes and means that could harm him with the permission of Allah. Hence, Allah ordered him to continue seeking Allah's refuge, reciting the supplications of morning and evening and avoiding a sick patients if there is no violation of Islamic ruling. The Prophet (Sallallahu Alaihi wa Sallam) said: Do not put a patient with a healthy person.” He also said: “One should run away from the leper as one runs away from a lion.” Also the Hadith reads: “Whoever hears that there is an outbreak of plague in some land, he should not go to that land, and if the plague breaks out in the land where one is already present, one should not run away from that land, escaping from the plague.
All the above details are mentioned by many great scholars.
As for the Hadith: The fever is from the heat of the Hell. Muslim scholars have two opinions. A group understands the Hadith in its true sense and says that the heat of fever that is found in the body is actually from the heat of Hell. Allah wants to put it in the body of the person for deep and great reasons, He knows best.
They support their opinion through another Hadith that reads: Fever is the portion of a Mu’min from Hell fire. Sheikh al-Albani graded it as sound and Imam Ibn Hajar graded the chain of the Hadith as acceptable.
The second group is of the opinion that the Hadith is in a figurative meaning consisting of a metonymy and similar expression so they say that heat of the fever is similar to the heat of Hellfire to warn people of the Hellfire.
Imam Ibn Hajar said that the first opinion is the preponderant one.
Some scholars say in their response to non-Muslims regarding the last Hadith, it is possible that Hell has some emissions that affect the climate of the Earth and on the lives of the people as well.
Allah knows best.