Gemara

Shmuel would drink water in a Nochri's house. Even though they are not concerned enough for snakes, they cover water for cleanliness.

Shevuos 15b (Beraisa): Tehilim 91 is the Shir of (Hash-m's protection from) Pega'im (damaging angels) - "Yipol mi'Tzidecha Elef". Some call this the song of plagues - "v'Nega Lo Yikrav b'Ohalecha."

R. Yehoshua ben Levi would say this before going to sleep.

Pesachim 112a (Beraisa): If one puts food or drink under a bed, even if it is covered in an iron container, Ru'ach Ra'ah dwells on it.

(Beraisa): One may not drink water on Tuesday night (without a Ner) or Shabbos night. If he does, he is liable for his own death (due to Ru'ach Ra'ah).

Chulin 16b: A Beraisa permits Shechitah with a reed.

(Rav Papa): Only a Simona reed (a particular species) may be used.

Bava Basra 58a: Under a Chacham's bed, only his shoes are found. Under an ignoramus' bed, a whole storehouse is found.

Rishonim

Rif and Rosh (Avodah Zarah 10b and Sof 2:13): The Yerushalmi says that we must be concerned for everything that people are concerned for... one may not put a cooked food under a bed. `

Rambam (Hilchos Rotze'ach 12:5): One may not put a cooked food under a bed, even if he is engaged in a meal, lest something damaging fall in, and he will not see it.

Poskim

Shulchan Aruch (YD 116:5): One may not put a cooked food or beverages under a bed, for Ru'ach Ra'ah dwells on it.

Toras Chayim (Shevuos 15b): Rashi says that R. Yehoshua ben Levi would say Shir Shel Pega'im at night, when Mazikim are able to harm. However, the Kolbo and Rema (OC 231:1) say to say it also before sleeping during the day. Sleep is a taste of death, and there is no Neshamah to ward off Mazikim. This is why we must wash hands in the morning, and why there is Ru'ach Ra'ah on food under the bed, like in a room with a Mes.

R. Akiva Eiger: Shevus Yakov permits the food b'Di'eved.

Chayei Adam (2:2): The Beis Yosef cites Ba'al ha'Itur, who says that Rashi forbids Shechitah with other reeds due to chips that fly off, which can disqualify the Shechitah. The Yerushalmi forbids due to Ru'ach Ra'ah; if so, b'Di'eved it is permitted.

Question: The Gemara and Poskim are very stringent about this and similar matters in this Siman!

Answer (Shevus Yakov 2:105): Surely food under a bed is permitted b'Di'eved. A Chaver may not eat in an Am ha'Aretz' house due to what is not Metukan (Tevel). A whole storehouse is found under an Am ha'Aretz' bed. Rashi explains that this refers to food and Kelim. Chachamim were not concerned lest a Chaver eat what was under a bed, for it is only l'Chatchilah. Also, no Posek forbids these matters b'Di'eved.

Rebuttal (Birkei Yosef Shiyurei Berachah 10): When Ru'ach Ra'ah is on a reed, b'Di'eved we permit what it cut. Here, the Ru'ach Ra'ah is on the food itself! If one eats a peeled garlic that was left overnight (Nidah 17a), or drinks water on Tuesday night, he is liable for his own death, due to Ru'ach Ra'ah! This shows that Ru'ach Ra'ah forbids even b'Di'eved. The Gemara did not say 'one may not put food under a bed', rather, 'Ru'ach Ra'ah dwells on it', to show that even b'Di'eved it is forbidden. Bechor Shor (6) says that we forbid Shechitah with other reeds lest a chip separate (into the meat) and it will end up in one's mouth, and the Ru'ach Ra'ah will harm him. He permits b'Di'eved, for 'perhaps no chip remained, and if it did, perhaps it will fall off; we do not forbid b'Di'eved due to Ru'ach Ra'ah.' He means that we do not forbid such cases, but we forbid when Ru'ach Ra'ah is on the food itself.

Defense (Kaf ha'Chayim 44): Perhaps not every Ru'ach Ra'ah is the same. The Ru'ach Ra'ah on a peeled garlic forbids b'Di'eved, but that on food under a bed does not! We are concerned b'Di'eved only when Chachamim said 'he is liable for his own death.' Many Poskim rely on Shevus Yakov. We may be lenient if it was on a tiled floor, or for a big loss or a raw food even on an earthen floor. The Ben Ish Chai (2 Pinchus 14) forbids even a raw food b'Di'eved. Had he seen all these Poskim, he would have permitted!

Minchas Yitzchak (2:68): Teshuras Shai observes that we permit buying cooked foods from Nochrim only because they are careful not to leave them exposed (Avodah Zarah 30a). There was no concern lest they were left under a bed. This shows that we are not concerned for this regarding Nochrim.

Note: This proof assumes that food left under a bed is forbidden b'Di'eved.

Binas Adam (63, on Chachmas Adam 68): The Gemara connotes that we are concerned even for raw foods. Once, a radish was put under a bed, and the Gra commanded to dice it and throw the pieces in a place where no one will find them. Nowadays, since many transgress this, "Shomer Pesa'im Hash-m" (Hash-m guards people who do not know to guard themselves) applies. Perhaps the (correct) text in Pesachim does not mention food at all, for the Rambam, Semag, Rif, Rosh and Ra'avad cite only the Yerushalmi. Perhaps the entire Beraisa was not in their texts, for they do not mention liquids. If so, only cooked food is forbidden. The Gra was concerned for our text.

Tzitz Eliezer (17:35): Once, a man slept on a box, and the Gra commanded to throw the food that was underneath to the river. We could say that what was in a person's pocket is Batel to him. Most Poskim permit b'Di'eved in every case.

Tzitz Eliezer (14:2): We cannot learn from this Isur to forbid bringing food into a bathroom, which has no source in Shulchan Aruch or Poskim. The Be'er Heitev forbids only eating there. Ri Algazi (Shalmei Tzibur, Hilchos Netilas Yadayim) found no source to forbid even from Kabalah, and he was as expert in Zohar as in Nigleh. However, R. Chaim Palagi (Lev Chayim 66) leans to equate these. R. Yosef Chaim Zonenfeld (Salmas Chaim 3:9) holds that there is no Isur to bring food into a bathroom, but it is improper due to Nekiyus ha'Da'as (refined conduct). Perhaps this is only for food, but not for pills. Afarkasta d'Anya says that even if we were concerned for Ru'ach Ra'ah, this applies to latrines in the field like in the days of Chazal, but not to our bathrooms. Perhaps our bathrooms have no Tum'ah at all (see Chazon Ish Mo'ed 17). In any case, if it is not hard, it is better to be stringent and not bring in even pills.

Yabi'a Omer (4 OC 5): We cannot learn from one Ru'ach Ra'ah to another. In one way, Tum'ah of a bathroom is greater than that of sleeping. One who left a bathroom must wait the time to walk a half Mil (9 minutes) before having relations, but we do not find that one who woke up must wait! R. Zalman held that wearing gloves while sleeping shields from Ru'ach Ra'ah. All the more so, a covered Keli protects water from Ru'ach Ra'ah in a bathroom! Not all agree to R. Zalman, but in any case a cover limits the Ru'ach Ra'ah. One may wash with water that was in a bathroom in a covered Keli. Most Poskim say that there is no Ru'ach Ra'ah on one's hands after he leaves a bathroom; R. Chayim Palagi, based on the Zohar, is stringent. It is proper to be stringent l'Chatchilah not to take food into a bathroom even if it is covered and wrapped. B'Di'eved it is permitted. If one has no place to leave it outside, this is like b'Di'eved.

Ben Ish Chai (Od Yosef Chai Toldos 6, Sod Yesharim, b'Sof Rav Pe'alim 4, Siman 5): If a Yisrael touched food before washing in the morning or after using the bathroom, or if food was under a bed, the food is Tamei. A liquid should be thrown out. A solid is rinsed three times. One should peel off a layer if possible. If David put Levi's food under a bed, one cannot forbid another's property. If Levi's wife, son or slave did so, we could say that their hand is like his (it is as if he put it there, and it is forbidden).

Salmas Chaim (1:12): If David put Levi's food under a bed, since Beis Din cannot force one to pay for damage that cannot be perceived, Ru'ach Ra'ah does not cling to it.

Yabi'a Omer (1 YD 10): R. Chayim of Volozhin says that the Gra was stringent for himself if food was under a bed for a short time (in the name of Rav Zundel - for a short time during the day), but he would feed it to others.

Shemiras ha'Guf veha'Nefesh (14:5): Cases occurred, and the Chazon Ish said to give the food to Talmidim or Aniyim without informing them. If a box of food was under a bed, the Chida and Malbim say that it suffices to immerse the box three times. Ya'aros Devash says that this does not help.