Nedarim 41a: Said Rabbi Alexandri in the name of Rabbi Chiya bar Abba, and some say Rabbi Yehoshua ben Levi: When the end of a person’s life comes, anything can overpower him, as Kayin said to Hashem, “Anyone who finds me will kill me” (Bereishis 4:14). Rav derived this from a different posuk: “They stood for Your judgment today, for all are your servants” (Tehillim 119:92).
The Mefaresh explains: Anything can overpower him – even a fly or a mosquito.
The Ran explains: Once the verdict of Your judgment is that the person’s time has come to die, all things are Your servants to carry out the sentence.
נדרים מא ע”א: וא״ר אלכסנדרי א״ר חייא בר אבא, ואמרי לה אריב״ל: כיון שהגיע קיצו של אדם הכל מושלים בו, שנאמר: (בראשית ד) והיה כל מוצאי יהרגני. רב אמר, מן הדין קרא: (תהלים קיט) למשפטיך עמדו היום כי הכל עבדיך.
One maggid shiur explained this Gemara with the Mefaresh’s comment by saying that the fly or mosquito might bite him and infect him with a deadly disease. But one of the listeners spoke up and said, “I can explain it based on my own experience. I once set out in my car and said Tefilas Haderech, and as I said the words, ‘Save us from dangerous animals on the road,’ I wondered to myself, why do we still say these words nowadays? Usually we’re traveling inside a car and the animals, if there are any, pose no risk. About a half hour later, I got my answer. A small fly entered my eye as I was speeding down the highway, and I lost control of the car and crashed. Miraculous, I emerged with only minor injuries. Then I understood that sometimes, the ‘dangerous animals’ can mean flies. Probably that is the meaning of the Gemara in Nedarim too: when there is a decree of death upon a person, even a fly or a mosquito can overpower him.”
Source: Meoros Hadaf Hayomi, Nedarim 41a (v. 3 p. 160)
[The listener’s explanation seems more correct than the maggid shiur’s. The Gemara is saying that anyone can be Hashem’s messenger to carry out a death sentence, even those creatures who aren’t usually harmful. If the meaning is as the maggid shiur said – that a mosquito infects him with malaria – that is the usual way of getting malaria. That does not illustrate how Hashem uses an unusual messenger. According to the listener, however, flies do not usually cause car accidents, so his story shows that anything can kill if that is Hashem’s decree.
Why does the Gemara bring a proof from Kayin? Because when Kayin said, “Anyone who finds me will kill me,” he was not afraid of any human avenger. Hevel had no children, and Adam and Chava would not kill their own son. Clearly, Kayin meant an animal sent by Hashem, because he thought that Hashem had decreed death upon him.]
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