How apt is the saying:
If you do not know, that is a calamity; but if you do know, the calamity is far greater.

Many people imagine that this hadith contradicts what physicians affirm—that the fly carries microbes on its limbs, and that when it settles on food or drink those microbes cling to it. In truth, the hadith does not oppose the physicians in this; rather, it supports them, for it tells us that one of the fly’s wings contains disease. What it adds, beyond their knowledge, is the statement: “and the other wing contains the cure.” Of this they have no knowledge; therefore, they are obliged to accept it if they are Muslims, and if they are non-Muslims possessed of intellect and learning, then at least to suspend judgment. Sound knowledge teaches that lacking knowledge of a thing is not the same as knowing that the thing does not exist.

We speak thus on the assumption that modern medicine has not yet borne explicit witness to the hadith’s truth. Medical opinions have differed on the matter; I have read many articles in various journals, each side supporting its conclusion, whether affirmative or dismissive. As believers in the authenticity of the hadith and in the Prophet ﷺ—who “does not speak from his own desire; it is but revelation revealed” (Qur’an 53:3-4)—we are not overly concerned with the hadith’s confirmation from the standpoint of medicine, for the hadith is a self-standing proof that needs no external support. Still, faith is strengthened when sound science coincides with an authentic hadith. For this reason, it is useful to relay to the readers the gist of a lecture delivered by a physician at the Islamic Guidance Society in Egypt concerning this hadith. He said:

“Flies alight upon filthy substances that abound with the germs from which various diseases arise. They carry some of those germs on their limbs and ingest others, and from this there forms within their bodies a toxic substance…"