Bukhari

Hadith Commentary: This itself is proof that contagion has no reality.

If it is said that the camel contracted mange from another camel, then from whom did that camel contract it? In the end, this would necessitate an infinite regress, which is impossible, or it would have to be said that one camel developed mange by itself. You (sallallahu alayhi wa sallam) presented such a rational and logical argument that the lame horse of the physicians cannot even walk in front of it.

Now, what is observed—that some diseases, such as plague or cholera, spread from one settlement to another, or pass from one person to another—this does not prove that the disease was transmitted. Rather, by the command of Allah, it also arose in that other settlement or person. The evidence for this is that in the same household, some die from plague while others do not, and in the same hospital, doctors and nurses treat plague patients, yet some doctors and nurses contract the plague while others do not. If contagion were real, then all would contract it. Therefore, the truth is what the Truthful Informer (sallallahu alayhi wa sallam) has stated. However, even Plato does not have a cure for delusion. (Waheedi)

Hadith Commentary:
(1)
The Messenger of Allah sallallahu alayhi wa sallam, in response to the objection of a Bedouin, explained that everything is from Allah Ta'ala, because He alone is the true effective cause.
It is entirely incorrect to say that a mangy camel caused the rest of the camels to become mangy.
If that were the case, then all the camels would have become afflicted, whereas in a herd there are many camels that remain safe from this disease.

(2)
If it is said that the first camel also contracted mange from another camel, then the question arises: from whom did that camel contract it? Ultimately, this would lead to an infinite regress, which is impossible, or it would have to be said that mange appeared in one camel by itself, and the One who brought it about was Allah Ta'ala.

(3)
One should avoid the causes of disease, as the Prophet sallallahu alayhi wa sallam forbade mixing a sick camel with a healthy one.
Now, what is observed—that diseases like plague and cholera spread from one city to another, or from one person to another—does not prove that the disease itself is inherently contagious and transferred to another. Rather, it is by the command of Allah Ta'ala that it appears in another city or person as well. Thus, it is seen that in one household, some people succumb to plague or cholera, while others remain safe, and in the same hospital, doctors treat patients of plague and cholera, yet nothing happens to them. If the disease were inherently contagious, then all would be affected by it. Therefore, the truth is what the Messenger of Allah sallallahu alayhi wa sallam has informed: that no disease is inherently contagious. However, one must certainly avoid such means and causes that may lead to the disease.
And Allah knows best.