In the name of Allāh,
the Beneficent, the Merciful.
Peace and Blessings of Allāh on Mohammad.
Allāh–the Glorious and the High,
Lord of the worlds
Mohammad–who brought the world
to our feet and eternity to our arms.
According to Bokhari Vol. 7 hadith #673, Prophet Mohammad says: “If a fly falls in the vessel of any of you, let him dip all of it (into the vessel) and then throw it away, for in one of its wings there is a disease and in the other there is a healing (antidote for it) i.e. the treatment for that disease.” (And the footnote to this hadith states that modern medical investigation has confirmed the truth of this saying of the Prophet. (And to know the Prophet taught this in the Seventh Century).
However, Answering Islam critic tries to denigrate the Prophet for this saying, noting that flies carry various deadly diseases. Conveying that this instruction from the Prophet is harmful.
These diseases such as tuberculosis and cholera carried by flies are from fecal matter (and in which the person has the infection) and rotting food and other filthy stations. (According to the Internet, singularly, a fly travels between 1-2 miles from its station, and about 20-25 miles in packs. It is doubtful that a fly coming from 20 miles away, making some five stops, and in inferno-temperature Arabia and sandy environament would still have TB or cholera bacteria on it).
(Whereas this does not mean that flies are to be tolerated in one’s house or any other place especially where there is food), unless you have an open fecal pit in your house or neighborhood or rotten food or other fly-attracting place(s) it is unlikely that the fly in your house would have the tuberculin or cholera or other dangerous bacteria.
Given that in Seventh century Arabia water may have been scarce or hard to come by, this information from the Prophet must have been valuable to the people.
The question to Answering Islam critic is: Given that you now know that submerging the fly into the liquid neutralizes the pathogen on its wings, if you were in an area (say Africa) where water is extremely scarce and there was a fly in your only vessel of water, and whether you suspect the fly may have the TB or cholera bacteria or not, will you dunk the fly and toss it away and drink the water, or will you toss the water?
(For our response to charges by Answering Islam see Answering Islam critic).