Gemara

9a (Beraisa): If one saw a bird pecking at a fig, or a mouse making holes in a watermelon, we are concerned that there was already a hole there from a snake, so one may not eat the food.

10a (Mishnah): Water, wine or milk is forbidden if it was exposed long enough for a snake to come from under the handle, drink, and return.

59a: The hind legs of a deer were cut. Rav planned to lightly roast it. Shmuel was concerned lest a snake bit it where the legs were cut off. He put it in the oven to fully roast it. It started crumbling (due to venom inside).

Avodah Zarah 30a: Rav would not drink water in a Nochri's house, for they are not careful enough about Giluy. He would drink in a (Yisraelis) widow's house. She conducts like she did in her husband's lifetime.

Shmuel would not drink water in a widow's house. She has no fear of a husband, so she is not careful enough about exposure.

30b (R. Chiya bar Aba): If part of a fig remained on the tree, we are not concerned that a snake bit the part left on the tree and poisoned the fig.

(Beraisa - R. Eliezer): One may eat figs and dates at night (when he cannot check them for holes), without concern - "Shomer Pesa'im Hash-m." (Hash-m guards fools who do not know to guard themselves.)

(Rav Safra): One may not eat from a cut watermelon that was exposed, even if nine ate from it. A case occurred in which a 10th ate, and died.

Beitzah 7b (R. Yosi ben Sha'ul): If a grated garlic was exposed, it is dangerous.

Mishnah (Terumos 8:6): If there is a hole in a fig, grape, gourd, watermelon, or cucumber, even if the fruit is like a Kikar (a weight of 3000 Shekalim), whether it is big or small, detached or attached, as long as there is moisture, it (the entire fruit) is forbidden. An animal bitten by a snake is forbidden

Rishonim

Rif and Rosh (Avodah Zarah 10b and 2:13): R. Chiya bar Aba said that we are not concerned if part of a fig is left on the tree. This is like R. Eliezer. If a grated garlic was exposed, it is dangerous. If there is a hole in a fig, grape, gourd, watermelon, or cucumber, even if the fruit is like a Kikar (Rif - in a cask), whether it is big or small, detached or attached, as long as there is moisture, it is forbidden. An animal bitten by a snake is forbidden.

Piplulei Charifta (40): If we are concerned for holes in figs, we should be concerned if part of a fig was left on the tree! The Gemara said that R. Chiya holds like R. Eliezer, i.e. unlike the Stam Mishnah in Terumos. However, the Rambam and Tur (also) rule like both of these! Really, there is no contradiction. If part of a fig is left on a tree, it is a Safek whether a creature ate from it. Even though if we saw a non-venomous animal bite a food, we are concerned lest a venomous animal bit if before (Chulin 9a), there is different, for we saw an animal make a hole. Also, people regularly eat figs missing the stem, therefore we are not concerned.

Rambam (Hilchos Rotze'ach 11:8): If a grated garlic or cut watermelon was exposed, it is forbidden. The same applies to all similar cases.

Rambam (14), Rif and Rosh (ibid.): If a cut watermelon was exposed, even if nine ate from it, one may not eat from it.

Rambam (16): If a dough was kneaded with exposed water, we burn it, even if it is of Terumah. Even if it was baked, the bread is forbidden.

Me'iri (Avodah Zarah 30a): The Rambam did not bring the Isur to eat food of people who are not careful about Giluy. The Gemara said that Rav and Shmuel did not drink water of widows or Nochrim. It did not forbid doing so! The Yerushalmi says that we are not concerned if a person goes in and out.

Note: The Bavli forbids if it was unguarded long enough for a snake to come from under the handle, drink and return! Perhaps the Me'iri proves that this was not followed everywhere (e.g. in Eretz Yisrael), therefore we may be lenient where there is no concern.

Me'iri (DH bi'Mkomos): The Yerushalmi says that one should not be too exacting about such matters. Rather, he relies on "Shomer Pesa'im Hash-m."

Poskim

Shulchan Aruch (YD 116:1): Chachamim forbade liquids that were exposed, lest a snake drank from them and put in venom. Nowadays snakes are not common among us, so they are permitted.

Taz (115:10): Semak says that according to R. Tam, who forbids cheese of Nochrim due to Giluy, it is forbidden even nowadays. We permit water nowadays because there was no other reason to forbid it. All know that it is forbidden where there is concern for Giluy. The Gemara gave other reasons to forbid cheese. If we would permit it where there is no concern for Giluy, perhaps people will think that it was forbidden for different reasons, and permit it even where there is concern for Giluy.

Pischei Teshuvah (1): The SHLaH says that in any case, one who guards his soul will distance from this. The Tur wrote all of these laws. The Gra was very careful about this. One who wants to be careful should see all the laws in the Tur.

Question (Pri Chodosh): Tosfos (Avodah Zarah 5b DH veha'Tanan) said that even though we have no Korban Pesach nowadays, we still learn laws of Pesach 30 days before Pesach, even though the enactment was primarily for the Korban. Likewise, Melachah on Erev Pesach in the afternoon was forbidden due to the Korban, nevertheless it is forbidden even nowadays. All Poskim except for the Ba'al ha'Ma'or agree to this! Nochri cheese is forbidden even where they curdle it using herbs, because it was forbidden through a Minyan. Even though the reason does not apply, another Minyan is required to permit it. However, Chachamim forbade sending gifts to Nochrim on the day of Nochri festivals, yet Rav Yehudah and Rava sent gifts to Nochrim whom they knew did not serve idolatry (so the reason did not apply)! Nochri fish oil was forbidden lest they mix in wine, but it is permitted where wine is more expensive than fish oil, so the reason does not apply!

Answer (Pri Chodosh): We must say that Chachamim forbade sending gifts only to idolaters, and fish oil was forbidden only where wine is cheap. The Ran (Avodah Zarah 31a DH Garsinan) says that Tosfos (57b DH Le'afukei) holds that a new Minyan is needed only to permit those for whom the reason for the decree applies. This is an excessive leniency. Perhaps Rav Yehudah and Rava knew that Chachamim never forbade business with Nochrim who do not serve idolatry. What is the source to be lenient about all decrees whenever the reason does not apply?! Tosfos is difficult; the Ran's opinion is primary. However, elsewhere Tosfos says that nowadays we are not concerned for Mayim Acharonim or washing before giving bread to a child, because Sodom salt and Ru'ach Ra'ah are not common among us, just like we are not concerned for Giluy and Zugos (doing things in even numbers) nowadays. Perhaps the decree about Ru'ach Ra'ah never spread throughout Yisrael. Similarly, Sodom salt was common only near Sodom. Concern for Giluy was only where snakes and scorpions are common. Zugos affects only one who is particular about it. Nochri cheese was forbidden even where they curdle with herbs, for they could come to curdle with stomachs of Neveilos. Learning Hilchos Pesach before Pesach, and not doing Melachah on Erev Pesach, were enactments for all of Yisrael. They apply even after the reason does not. Tosfos (Avodah Zarah 35a DH Chada) says that since we hold that Nochri cheese was forbidden due to Nikur (perhaps a snake ate from it), it is permitted where snakes are not common. Tosfos in Beitzah (6a DH v'Ha'idna) says that Chachamim made Yom Tov Sheni like a weekday regarding burial, but retracted due to Chavrei (a caste of evil Nochrim), if they see us bury on Yom Tov Sheni they will force us to work for them. Nowadays there are no Chavrei, so it is permitted. We do not need another Minyan, for the reason no longer applies, like we say about Giluy. This is wrong; what I wrote is primary.

Magen Avraham (690:21): If something was decreed amidst a Minyan, even if we know the reason and it does not apply any more, another Minyan is needed to permit it. The second Beis Din must be bigger in Chachmah and Minyan. If something is only due to a concern, and the concern passed, another Minyan is not needed, like it says in YD 116.

Kaf ha'Chayim (3,9): A case occurred in which an entire family became dangerously sick from an uncovered food. People say that it is because a spider left its spit on it. Even nowadays we should not drink at night nor leave cooked foods exposed due to spiders. If one did not cover it, we do not forbid the food. We rely on "Shomer Pesa'im Hash-m." Even without concern for poison, one should cover all foods on which ants or flies might come, or that mice might eat from them.

Kovetz Shi'urim (Kesuvos 136): Even though regarding danger we do not rely on "Shomer Pesa'im Hash-m", one may do what is normal. It is as if he cannot be careful, therefore Hash-m will guard him. This is why we may eat figs and dates at night (when one cannot check for holes), without concern.