The Chachamim prohibited a number of practices due to the perceived danger they entailed. In their words, "Chamira Sakanta MeIssura" ([The chance of] danger is more serious than sin). As such, they prohibited drinking water left uncovered, known as "Mayim Megulim" (uncovered water, colloquially referred to as "Gilui"), as a snake may have drank from the water and replaced it with venom. The Gemara details several Halachot that are pertinent to avoiding Mayim Megulim, which beverage it applies to, and external factors that should also be taken into account. In general, the Rambam writes, one who violates the health injunctions of the Chachamim flagrantly and says that he's willing to endanger himself and doesn't care what other people say is punishable by Makkat Mardut.
Many Rishonim note how, nowadays, snakes are not so common, so these Halachot do not apply. True, this is a takanat chachamim, and should be unequivocal (Lo Pelug Rabbanan), the standard explanation for the leniency is that they only prohibited water left uncovered in areas where snakes are common. In areas where snakes are uncommon, the prohibition never applied in the first place. On the other hand, the Chachamim of various locations and generations have noted that it should apply in their locales, such as Morocco, where snakes and a variety of other critters are always hanging around the drinking water, and Eretz Yisrael. At the same time, the Poskim limit the prohibition to drinking and not other activities with the water, such as washing hands or feeding to animals. Indeed, the Vilna Gaon is said to have been strict about this even in Chutz LaAretz.
This is only a sampling of examples. See the 8th Perek of Terumot, Rambam (Hilchot Rotzeach 11:5-16), Rashba (Torat HaBayit Bayit 3 Shaar 4), Tur (Yoreh Deah 116:1), Pri Chadash, Darkei Teshuvah (ad loc), and the Kof-K's article on the matter for the comprehensive presentations.
Where Gilui Does Apply
- The danger of leaving beverages uncovered applies on a most basic level to water, wine, and honey. Some say it applies to tzir or miryas (juice/brine from fish or meat, for example).
- Yayin Mazug: diluted wine.
- Wine that has only begun to vinegar.
Where It Doesn't Apply
- Yayin Mevushal: cooked wine.
- Yayin Toses: within the first three days of fermentation.
- Sweet, bitter, and sharp wines are not subject to gilui, unless they are classified as Chamar Medinah.
- Fruits with a moist inner flesh are subject to gilui, but some, such as figs and grapes are permitted, unless they have bite marks.
- Water that was used to soak food and taste changed because of it is not subject to gilui.
- Beverages left in refrigerators and water in an urn are not subject to gilui.
- The suspicion applies both day and night.
- Beverages left unattended for as long as it takes for a small snake to slither out from under the handle of the container, take a sip, and return, becomes prohibited.
- If one is awake near an uncovered container of one of these beverages, there is no issue of gilui, but, if he's sleeping, his guard does not protect the beverages from the issur of Mayim Megulim. However, a consistent drip of water from one container into a lower one would at least permit the lower one and perhaps also the top one.
- If the wine is stagnant in a closed or deep pit, it remains permitted, as long as one ensured there was nothing else inside and there is no hole. If there is a hole, depending on the size, it could be problematic. Similarly, a stream of water is not subject to the prohibition.
- Bediavad, one may not do anything with mayim megulim and must discard it, even if he already cooked food with it. Therefore, one may not feed it to his or his fellow's animals or wash himself or anything else with it. Even a barrel that was left uncovered and then drunk from by people who did not get sick afterwards may not be drunk from further, because venom from snakes of different ages has a spectrum of potency and density, so it could have sunk further down.
- Rambam (Hilchot Rotzeach 11:5)
- Tosafot Avodah Zarah 35a. The Rashba (Torat HaBayit Bayit 3 Shaar 5) and Tur (Yoreh Deah 116:1) write comprehensive lists of the rules of Gilui and conclude with this point. The Shulchan Aruch (Yoreh Deah 116:1), on the other hand, doesn't even write the details and just says that it's no longer applicable.
- Tosafot Avodah Zarah 35a, Tur (Yoreh Deah 116:1 at the end). See Taz, Biur HaGra, Pri Chadash, Mateh Yehonatan, Kaf HaChaim ad loc. and Shu"t Minchat Asher 3:22
- The Pri Chadash (Yoreh Deah 116:1) argues that snakes are common in the west (Morocco), and they're not too uncommon in Yerushalayim and its surroundings. He recommends every Yarei Shamayim be strict. In fact, he says, one year a whole family in Egypt got sick because of mayim megulim. Therefore, the Pri Chadash reintroduces the details of mayim megulim into the Shulchan Aruch. The Chida (Shiyurei Beracha Yoreh Deah 116:3) testifies that the minhag is to be machmir in Eretz Yisrael like the Pri Chadash. The Erech Lechem (Yoreh Deah 116:1), Chesed LeAlafim (Yoreh Deah 116:1), Kaf HaChaim (Yoreh Deah 116:6), Mekor Chaim HaShalem (264:2) and other poskim echo their position. The Zivchei Tzedek does testify that snake poisoning had been an issue in Baghdad, but Shomer Petaim Hashem. (Kaf HaChaim 116:3) Of course, he adds, if there is a bug issue, one must cover the water regardless.(Kaf HaChaim 116:9) Nehar Mitzrayim vol. 2 page 178 says that it's worth being machmir given the ease of fulfilling all opinions. Magen Avot (Yoreh Deah 116:1) writes how in Morocco it was common to find all sorts of things in the water, so it was necessary to be makpid. Remarkably, Rav Chaim Ibn Attar (Pri Toar 116:2) testifies that growing up in Salé, Morocco, there were no snakes. Even when he moved to Fes, where there were snakes and scorpions everywhere, but there was never an incident of snake (not a scorpion) leaving venom behind and killing someone. Everyone lived in harmony with them. Only because an elder told him that it had happened once, he decided that one must be concerned. His ruling was limited only to drinking mayim megulim, not to the other activities one should not do with the water, such as feeding it to animals or washing oneself or other things, as there were no testimonies of those ever being a problem. (The Chida in Shiyurei Beracha ad loc 4 notes how he had heard of this ruling from the Pri Toar) He also says one must be concerned for the scorpion bites in foods. Rav David Pardo (Mizmor Ledavid ad loc, cited in the footnotes of the Pri Toar and Chesed leAlafim ad loc) notes how snakes are only common in the fields and not in the city, and, even then, their presence is obvious due to their pungent smell similar to that of donkeys. As such, one must be concerned for nikkur (bites in fruit), but not gilui except in the houses where the smell is established, even though the snakes don't cause any harm to humans. Even though we believe the concerns of the Chachamim to be valid, he says, he applies Shomer Petaim to be lenient. Additionally, the Gemara indicates that cats are immune to snake venom and eat snakes, so the fact that they had cats could be a protective defense mechanism against snakes in general.
- Pri Toar (ibid), Ben Ish Chaim (Shanah II Pinchas 9)
- Maaseh Rav 95, Pitchei Teshuvah (Yoreh Deah 116:1), Kaf Hachaim (Yoreh Deah 116:6). In the older editions, the Maaseh Rav adds that he said that there are other hidden reasons behind gilui in addition to the danger of snakes. In the Tosefet Maaseh Rav it quotes the Pe'at HaShulchan who details how the GRA said he keeps it because mayim megulim cause one to get distracted during Tefillah, and how the GRA went to incredible lengths to keep it. One time, in a storm, when his shamash lied and said that the water he brought was not meguleh, the GRA drank it and then remanded him after praying, as it clearly caused him to get distracted during his prayer.
- Chullin 10a and 49b, Rif Avodah Zarah, Tur (Yoreh Deah 116:1)
- See Beit Yosef ad loc who quotes how all three Amudei Horaah - the Rif, the Rambam (Hilchot Rotzeach 11:7), and the Rosh - pasken that Tzir is subject to gilui, but he thinks that's only true according to R' Shimon in Chullin 49b, whereas we hold like the Rabbanan, as the Rashba says. He opines that the Amudei Horaah were concerned for sakana even though it was minute chance. The Bach ad loc disagrees with the Beit Yosef, but the Pri Chadash ad loc argues back in his favor. Pri Toar and Shiurei Beracha ad loc bolster the Beit Yosef's view, as well.
- Avodah Zarah 30a. Tur (Yoreh Deah 116:1) pins it on the taste of wine, but the Pri Chadash argues that taste is irrelevant, as it's diluted with water! He interprets the Tur, therefore, to be referring to a case where wine is diluted with other liquids.
- Tur (Yoreh Deah 116:1) according to Nehardaei in Avodah Zarah 30a
- Avodah Zarah 30a. See [] regarding the definition of mevushal. See Beit Yosef and Erech Lechem (Yoreh Deah 116:1) regarding wine which has steam coming up out of it.
- Avodah Zarah 30b
- Avodah Zarah 30a, Rambam (Hilchot Rotzeach 11:10)
- Avodah Zarah 30b. See Talmidei Rabbeinu Yonah ad loc
- Rambam (Hilchot Rotzeach 11:9)
- Shu"t Avnei Derech 5:6. Note the well-written introductory survey of the issue.
- Rambam (Hilchot Rotzeach 11:11), Tur (Yoreh Deah 116:1)
- Chullin 10a
- Avodah Zarah 30a
- Avodah Zarah 30b. See Beit Yosef (Yoreh Deah 116:1), Taz, and Pri Chadash.
- Rambam Hilchot Rotzeach 11:13
- Rambam (Hilchot Rotzeach 11:16). See Pri Chadash (Yoreh Deah 116:1) and Shiurei Beracha ad loc. 6 for a discussion of the matter.
- Avodah Zarah 20b, Rambam (Hilchot Rotzeach 11:15).
- Rambam (Hilchot Rotzeach 11:14)