Gemara

(Beraisa): In Kelim, we may eat fish even without Simanim.

67a (Rav Huna): One should not strain (date) beer through a wooden strainer at night, lest a worm alight on the strainer and fall into the cup;

Since the worm (once) separated from the beer, it is considered "a Sheretz that swarms on the ground." (It is forbidden.)

Question: If so, one should not drink beer in a Kli, lest a worm alighted on the Kli and returned to the beer!

Answer: In that case, it is considered as if the worm never left the water. (It is permitted.)

Question: What is the source to distinguish?

Answer (Beraisa): "You may eat anything in water" permits bending down to drink from pits (even though one may ingest worms).

Question: Perhaps the worms alighted on the wall of the pit!

Answer: Since they never left the pit, they are permitted.

Similarly, if a worm in beer separated to the Kli wall, it is still permitted.

Support (for Rav Huna - Beraisa - Rav Chisda): "Every Sheretz that swarms on the ground" includes flies that were strained from wine.

Inference: Had they not been strained, they would be permitted.

Questions (Rav Ashi): If a worm separated to the outside of the date, or to the outside of the date pit, or from one date to another, what is the law?

These questions are unresolved.

Rishonim

Rambam (Hilchos Ma'achalos Asuros 2:18): One may bend down and drink water from a pit, even though he swallows some small Sheratzim.

Rambam (19): This is if the Sheretz never left the place it was created. If it left, even if it returned to the Keli or pit, it is forbidden. If it went to the wall of the barrel or pit and fell back into the water or beer, it is permitted.

Rambam (20): If one strained wine, vinegar or beer and ate the worms that were strained off, he is lashed even if they returned to the Kli after he strained them, for they separated. If he did not strain it, he may drink it.

Beis Yosef (YD 84 DH Kasav): The Rambam connotes that they are permitted before they separated. Hagahos Maimoniyos (6) says so in the name of Semag (and Sefer ha'Terumah).

Rosh (68): Some say that the Gemara permits to bend down and drink water from a pit, but not to drink from a Keli, lest a worm alighted on the wall of the Keli (and became forbidden). We could say that the Gemara merely gave a typical case, but one may drink also from a Keli. As long as we do not see that a worm alighted on the Keli, we are not concerned for this. The Gemara below connotes that whenever it would be forbidden if it separated, we are concerned for this.

Poskim

Shulchan Aruch (YD 84:1): Sheratzim that grow in water in Kelim and pits that do not flow are permitted. Therefore, one may bend down and drink from them, without concern lest Sheratzim come to his mouth.

Shach (1): The same applies to other liquids. The Rambam, Hagahos Maimoniyos and other Poskim say so.

Rema: One may not draw with a Keli and drink.

Shach (4): It is uncommon to separate to the brim of the Kli, but it is common to separate to the inside wall of the Kli. This forbids, for initially the worm was in the pit. We permit (only) when it separated with the liquid from one Kli to another, even if it separated to the (inside) wall of the second Kli. This is Revisei (where it grows), to be there with the liquid in a Kli. Therefore, the second Kli is like the first. One may not draw with a Keli from a pit and drink, but one may draw with a small Keli from a big Kli. The Rosh forbids when there is concern lest it separated to the wall of the Kli. Isur v'Heter connotes like this. The Levush forbids if one drew with a Kli and it separated to the wall. This connotes that if we did not see it separate, it is permitted! However, we can say that he means 'presumably it separated.'

Gra (3): The Gemara permits bending down to drink (but not to take water in a Kli and drink it).

Shach (5): Kol Bo says that similarly, one may not take water with his hand and drink.

Shulchan Aruch (ibid): If they left Revisei, e.g. they went to the back of the pit or the brim of the Keli on the outside, even if it returned it is forbidden. Stam, we are not concerned lest it left, but if they did not leave but they are on the inside wall of the Keli, they are permitted.

Shach (6): The wording (to the back of the pit or the brim of the Keli on the outside) is awkward. The Rashba says 'to the back of the Kli or the brim of the pit', which is better. It seems that even if the Keli has a thick brim, we are not concerned lest it separated to the brim (unless we saw this). This is unlike straining. Then, it is common to separate onto chips and pieces of straw.

Beis Yosef (DH v'Ha): We permit only if they did not separate. Rashi explains that if it separated to the wall, since this is Revisei, it is as if it never left. The Rashba says that even though the Gemara asked about a worm that separated to the outside of a pit (of a fruit), and did not resolve this, that is not Revisei. Revisei is only the hole in which it grew. Once it leaves its hole, this is like leaving the pit (ditch) in which it grew. Only the inside of the Kli or ditch is Revisei. If it went to the outside of the Kli or the brim of the pit, it is forbidden.

Taz (4): The inner wall of the Kli is considered Revisei.

Shulchan Aruch (3): If one strains water or other liquids, and there were worms or flies inside, even if they returned to it, they are forbidden, for they already separated. Therefore, if worms often grow in a liquid, one may not strain it at night with wood chips and straw, lashed the worms fall back into the Kli, and he come to drink them.

Shach (9): The Tur and Shulchan Aruch connote that surely if they returned to (i.e. afterwards we see them in) the Kli, they surely separated. The Gemara connotes that it is a Safek. We must say that the Tur and Shulchan Aruch agree. They did not elaborate to explain this.

Sifsei Da'as: Also Isur v'Heter connotes that it is only a Safek. This affects a Sefek-Sefeka (two doubts), e.g. if we see them dead in the water. Since it is a Safek whether they separated, and even if they did, perhaps they separated after they died (and this was not resolved), it is permitted.

Magid Shamayim (in Shulchan Aruch ha'Shalem, Hagahah 31): This is according to the Rosh, who permits worms that separated after death.

Rema: One may strain it through a garment or strainer, for they cannot fall from there to the liquid.

Beis Yosef (DH u'Mah): One may strain through a garment, for one casts off what separates onto it, and it cannot return to the liquid at night. The Rambam did not write the Isur to strain at night, for it is forbidden only through chips, and clearly it is forbidden for then the worms can return to the liquid. The Rashba mentioned the Isur regarding straw, like the Gemara. I do not know why the Tur mentioned the Isur without specifying that it is only regarding straw (or chips).

Rema: Similarly, one may pour liquid from Kli to Kli. Since they are always with the liquid, this is Revisei.

Gra (12, based on Darchei Moshe 2): We infer this, for the Gemara forbids only straining. This implies that the second Kli is like the first.

Beis Yosef (DH Kasav): The Gemara was unsure about separating from one date to another. Pouring from one Kli to another is different. The Rosh forbids if in one drew water in a Kli and it separated to the wall of the Kli, but we can say that this is when he drew from a pit. We can say that from Kli to Kli is Revisei.

Taz (5): I say that the Rosh is concerned lest the worm separate to the back or even the inside of the Kli before the liquid entered. Once there is liquid in the Kli, there is no Isur if the worm separates to the Kli.

Mishbetzos Zahav: The Taz does not distinguish drawing from a pit or from a Kli. It is forbidden only there is concern lest a worm separated before there was liquid in the Kli.

Sifsei Da'as (8 DH Kasav): The Pri Chodosh says that if one takes water from a pit with worms, and uses it to knead flour and bake bread, it is forbidden. Even if we are not concerned for separating to the wall of the Kli (it seems that the Mechaber holds like this, for he did not forbid this in Sa'if 1), later it separated onto the flour, and surely this is forbidden.

Note: The Rema forbids taking with a Kli from a pit, based on the Beis Yosef's Perush of the Rosh.

Bedek ha'Bayis: R. Yerucham says that if worms in vinegar separate to the air, this is called separating, but from vinegar to a cooked food is not.

Ha'Gaon R. David Feinstein Shlita (brought in Shulchan ha'Levi Birurei Halachah 21): New York City water contains small Sheratzim. When they are alive, a naked eye can see them moving even from a small distance. Even after they die, one can see white dots. Since we know that they were Sheratzim, they are forbidden. One can see them in a cup of water, so this is called that the Isur was recognized, so mid'Oraisa it is not Batel even in 1000. One must strain the water to remove them. Some permit like Sheratzim that formed in pits. This is wrong. Flowing water fills the reservoir, so it is considered a spring. Many worms are found in many neighborhoods in New York, therefore this is a common minority in the entire city. In such neighborhoods one must strain the water.

Rebuttal (Shulchan ha'Levi (R. Yisrael Belsky Shlita)): Tzadikim and Chasidim have never refrained from drinking the water without straining. We cannot say that they sinned, like Igros Moshe (YD 2:146) said (that previous generations did not transgress for eating tiny bugs that we can see through microscopes).

Note: Perhaps in the past there were no such worms, or they were too small to be seen. Had they been seen, presumably Poskim would have discussed this! Increased pollution or global warming could cause an increase in the size or number of worms.

Shulchan ha'Levi: The worms were first found in Prospect Park Lake, which is not a source of drinking water. Only about 5% of the white dots found are Sheratzim. Almost all are tiny pieces of paper or other matters that are not Asur, e.g. what came off pipes. The Poskim discussed worms in vinegar, i.e. when a film is seen on top of the vinegar. The worms in water are not considered recognizable. Sheratzim that are not born to parents are not forbidden. Scientists say that everything living is born to parents, but this is clearly unlike Chazal, who said that certain kinds of lice arise from sweat. (I.e. chicks develop in eggs, and the oxygen they need passes through the shell. Fetuses in animals get oxygen through the mother's blood. Lice eggs develop only amidst sweat or hair that they cling to, so this is not called being born (solely due to) parents.) New York City water is like pit water. Streams from the mountains bring water into great reservoirs. They are opened and closed constantly. Almost all the Sheratzim form from microscopic eggs in the pits. They add chlorine to the water, and it is turbulent. Together, these kill the Sheratzim.

Shulchan ha'Levi (9 and conclusion): If straining catches worms, and later they return to the water, they are considered worms that separated. A special strainer was developed in which the holes never widen or constrict (so it does not catch worms and later release them), but perhaps one may not use it on Shabbos. In summary, one need not strain the water, and straining is more likely to cause problems than solve them.

Kovetz Hilchos Pesach (of R. Avrohom Blumenkrantz, 5769, Chapter 29): Most Poskim in New York City say that one should assume that the water is problematic, and properly filter it. Many filters do not catch all the bugs. Even the fully reliable filters must be used and monitored properly. (The Sefer explains how to do so.) Special supervision is needed in restaurants. They need much water, and when a filter clogs it might be bypassed or hastily and improperly replaced by workers.