The Prophet (pbuh) said,

"If a housefly falls into the drink of anyone of you, he should immerse it completely in the liquid, then remove it, for one of its wings has the disease and the other has the cure."

This Hadith means that the fly carries on one of its wings a disease, and on the other a cure from the same disease. When a fly falls into a container (of food or drink), it puts forward the wing carrying the microbe, as a self-defense. Imam Ibn Hajar said in his commentary on the hadith that one of the scholars observed that the fly protects itself with its left wing, so it can be deduced that it carries the cure or the antidote on the right wing. So if the fly is immersed in whatever it falls on, the antidote will destroy the venom or the microbe with the will of Allah.

...A group of Muslim researchers in Egypt and Saudi Arabia carried several experiments on containers of water, honey and different juices. They exposed them to the flies. Then they immersed some flies in some of these containers. The microscopic examination showed that the liquids in which no flies were immersed were full of bacteria and viruses, while the others where the flies were totally immersed had none.