Gemara

For a sharp pain, one may drink water from which a dog drank at night. This should be taken from a place where snakes are not found, for the water was uncovered.

One who drank exposed water should drink a Revi'is of undiluted wine.

Chulin 10a (Mishnah): If water was exposed for long enough for a snake to come from a nearby hiding place, drink, and return, one may not drink it.

(R. Yitzchak brei d'Rav Yehudah): A nearby place is under the handle of the flask.

59a: Rav inspected a deer whose hind legs were cut, and ruled that it is Kosher. He planned to eat from it lightly roasted. 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. He said about Rav "no mishap will befall a Tzadik."

Avodah Zarah 30a: Water of R. Chilkiyah bar Tuvi's servant was exposed. He was sleeping right by it.

R. Chilkiyah: Snakes are afraid of people who are sleeping.

Rejection: This is wrong. Snakes are not afraid of sleeping people, not during the day or at night.

30b (Beraisa): One may not pour exposed water in a public road, nor sprinkle it in the house (to keep the dust from rising), nor use it to knead mud, nor give to his or another's animal to drink, nor wash his hands, feet and face with it;

Others say, he may wash only places where there are no cracks. (Drops remain in cracks, and the venom would burn through his skin.)

They argue about the back of the hand or foot, and cheekbones (there are no cracks, but the first Tana forbids them).

Rishonim

Rambam (Hilchos Rotze'ach 11:6): One may not drink water that was exposed, lest a snake or Zochalei Afar (creatures that crawl in the dust) drank from it, and the person will die.

Rambam (8): If the water was hot and steam was coming out, or it was dripping constantly, there is no problem of Giluy, for Zochalei Afar would not drink it.

Rambam (11): If a liquid for which we are concerned was exposed during the day or at night it is forbidden. This is even if a sleeping person was nearby, for Zochalei Afar do not fear sleeping people. It is forbidden if it was exposed long enough for a Rachash to come from under the handle of the Kli, drink, and return to its place.

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

Rambam (12:1): If any animal or bird was bitten by a snake or similar animal, one may not eat it, due to mortal danger. Therefore, if an animal or bird was found with its legs cut off, even if it is Kosher, one may not eat it until checking if it was bitten by Zochalei Afar. One roasts it in an oven. If the meat is like regular roasted meat, it is permitted.

Rambam (Hilchos Berachos 6:7): One may not wash his hands, feet and face with exposed water.

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.

Gra (1) and Pri Chodosh: This is not considered something forbidden through a Minyan (an assembly of Chachamim, which can be permitted only amidst a greater Minyan), for from the beginning it was forbidden only where there is concern for Giluy. Cooked wine and many similar things were never forbidden.

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

Bach (DH Yesh): The Tur wrote all the laws because snakes are still common in some places.

Pri Chodosh (1): They are common in western lands, and found but less common in Yerushalayim. In Mitzrayim, a host died from an exposed food, and all who ate from it became dangerously sick. Why should we risk our lives when it is easy to be careful?

Kaf ha'Chayim (3): 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.

Kaf ha'Chayim (6): The custom is to be stringent in Eretz Yisrael.

Birkei Yosef (2): The Yerushalmi says that we are concerned in Eretz Yisrael and Chutz la'Aretz. The Pri Chodosh says that this includes even Yerushalayim. Even though snakes never harmed there (Avos 5:5), we do not rely on miracles when danger is common. Others say that the Mishnah discusses only when the Mikdash stood.

Birkei Yosef (5): Exposed water is not Batel in a majority of good water, unless it is so much that the poison would be impotent.

Birkei Yosef (6): The Pri Chodosh forbids a dough kneaded with exposed water, even if it was baked. Fire does not detoxify venom. We learn from Chulin 59a. The Radak says that we learn the opposite! If it did not crumble in the oven, it is permitted! The Pri Chodosh says exactly like the Rambam. He rules unlike R. Nechemyah (Tosefta Terumos 7:13) who says that fire detoxifies venom. The Rambam (12:1) says that if meat does not crumble, it is permitted, but does not say so about bread. We must say that fire penetrates into meat that is roasting more than into bread that is baking.

Shmiras ha'Guf veha'Nefesh (Siman 44, Ha'aros): The Chazon Ish said that even nowadays we must be careful about Safek Giluy, just like the initial enactment. He said that a paper covering suffices, and we may be lenient about something left in a refrigerator. The Chazon Ish was particular not to leave exposed water for washing the hands in the morning.

Beis Yosef (OC 160): The Tur says that nowadays that we are not concerned for Giluy, one may use exposed water for Netilas Yadayim (washing before a meal). One might have thought that even though one may drink it, it is Pasul for Netilah, just like water with which work was done or changed color. Rather, since we are not concerned for snakes nowadays, the water is like before it was exposed.

Bach (DH Kosav ha'Rambam, cited by Mishnah Berurah 2): Even one who is particular not to drink exposed water nowadays may use it to wash before a meal. We do not say that for him it is like wastewater.