Gemara

(R. Eliezer ben Azaryah): If one has one Maneh (100 Zuz,), he should buy one Litra of vegetables. If he has 10 Manos, he should buy fish. If he has 50 Manos, he should buy meat. If he has 100 Manos, he should have meat every day.

(Rav): We should follow R. Eliezer ben Azaryah's teaching.

(R. Yochanan): Rav was healthy. He could conduct like R. Eliezer ben Azaryah. We are not healthy. One who has money should spend it on food.

(Rav Nachman): We are so weak that we should borrow money to buy food.

Megilah 21a (Beraisa): From the days of Moshe until R. Gamliel, people stood while learning Torah. After R. Gamliel died, people became weak, and people learned while sitting;

Yoma 82a (Mishnah): We do not make children fast on Yom Kipur, but we train them in the Mitzvah a year or two before (adulthood) so they will be used to Mitzvos.

(Rav Chisda): We train a healthy child two years before, and a sickly child one year before.

Horayos 13b (Beraisa): If one left (the Shi'ur in the Beis Midrash) for a need, he may return to his place.

(Rav Papa): That is only if he left to urinate. If he left to relieve himself (move his bowels), he was negligent (for not checking himself earlier if he needs to go), so he may not return to his place.

Nowadays, people are weaker, and even to relieve oneself is not negligence, so he may return.

Rishonim

Rashi (84a DH Amar Rav): The world kept deteriorating. Rav Nachman's generation was weaker than R. Yochanan's.

Tosfos (Mo'ed Katan 11a DH Kavra): Our Gemara says that a fish is good shortly before it spoils, and it is good to put it in cold water after roasting it. Nowadays, these are considered dangerous. Perhaps nature changed, just like the cures of Chazal are not good nowadays. Or, perhaps the rivers of Bavel improved fish.

Chavos Ya'ir (234 DH She'elah): I asked an expert doctor who is a big Chacham, how can doctors prescribe cures that depend on weights and measures for our weak generations? He said that it is a disgrace for a great doctor to do so. Really, according to the weakness of people to receive the cure, the herbs, roots and flowers used to make the cures are weaker. Man is nourished and fed from them. The strength of the small world depends on the big world.

Maharik (173): Do not say that previous generations were stronger and walked faster. R. Chanina said that five Mil separate Sedom and Tzo'ar (Pesachim 93b). The Gemara (94a) challenged him, for Lot traversed it between dawn and sunrise, and R. Yehudah said that one walks only four Mil in this time. The Gemara answered that he walked faster, for the angels were pressuring him. It did not say that people used to walk faster!

Me'iri (21a DH Kevar): One must stand for Kri'as ha'Torah. The Yerushalmi forbids even leaning. Just like the Torah witnesses given amidst fear, we must conduct with fear. Some Chachamim insisted that the Ba'al Korei, and one who received an Aliyah, not lean on the Bimah. However, perhaps we can rely on our Gemara, which says that after R. Gamliel died, weakness descended to the world.

Poskim

Shulchan Aruch (OC 326:1): One may not bathe the entire body (in hot water), even if it was heated from Erev Shabbos.

Binyan Tziyon (76 DH Teshuvah): The Chacham Tzvi (41) and Sidrei Taharah (197) did not permit Tevilah in hot water on Shabbos. Divrei Yosef said that the custom spread to do so, and no one protests. He struggled to justify the custom, and concluded 'this requires investigation.' He suggested that since it is bodily painful to immerse in cold water, Chachamim permitted to bathe in hot water. They permitted (OC 307:5) to tell a Nochri to bring hot water through a Chatzer without an Eruv for one who is in pain, to bathe in. Sidrei Taharah concluded that where the custom is to immerse in hot water, we do not protest, especially nowadays that weakness descended to the world and people cannot immerse in cold water. The Noda bi'Yehudah said that the entire decree was lest people warm water on Shabbos. Tevilah (of Nidah) is at the beginning of Shabbos night, so no one will immerse in water heated on Shabbos. We do not immerse in water that was in a Kli. Ultimately, he permitted only to heat the water some hours before Shabbos, so on Shabbos it will be only lukewarm. The bathhouse attendants do not fulfill this. He ignored this; it is better that they be Shogeg than Mezid. I say that the hot water that they add to a Mikveh is Batel to the Mikveh, so this is not included in the decree of bathing in hot water. I agree that it should be heated before Shabbos, so on Shabbos it will be only lukewarm.

Maharsham (3:140 DH v'Od): Nowadays the custom to heat Mikva'os spread. It was necessary, for weakness descended to the world and if we would not be lenient, many would refrain from immersing. Therefore, nowadays there is no suspicion (of heating the Mikveh on Shabbos).

Shulchan Aruch (616:2): An 11 year old boy or girl completes the fast (of Yom Kipur) mid'Rabanan for training in Mitzvos.

Mishnah Berurah (9): Nowadays no child fasts in the 12th year because everyone is weak. No child is considered healthy for Hashlamah (completing the fast) unless we know that he is healthy and can endure the fast. Eliyahu Rabah says that Hashlamah is not done even during the 13th year.

Maharsha (Shabbos 24a DH b'DH Ta'aniyos): The Beraisa says that one mentions the fast (Aneinu) in every Tefilah. It was not concerned lest one get sick and need to eat (and his prayer was false, for he did not complete the fast). The Ge'onim were concerned for this (and enacted to say Aneinu only at Minchah). This is like we say in general, that later generations are weaker.

Si'ach Yitzchak (259): Why do we say Selichos on fast days in the morning? Ta'anis 13a says that the last quarter of the day we request mercy! Perhaps it is because weakness descended to the world. The Avudraham says that we do not read the Haftorah at Minchah of fast days, for it is hard for people (they are weak from fasting). The Levush says that we do not say Ma'arivis (a Piyut) on Rosh Hashanah night, because people fast on Erev Rosh Hashanah. At Minchah, the Ma'amados (people who were Sheluchim of Yisrael to be present at the Korbanos Tzibur every day) did not bring a Sefer Torah, rather, they recited by heart (Ta'anis 4:3). The Bartenura explains that this is due to the fast. (They were weaker than us,) because they fasted four days (from Monday to Thursday). We read from the Sefer Torah at Minchah of fasts for rain, even though there are many fasts. This is because there is space between the fasts (they are on Mondays and Thursdays). Ma'amados fast four consecutive days. Selcihos for Erev Rosh Chodesh (Yom Kipur Katan) are at Minchah. Selichos said for what happened on that day (fast days) are unlike those due to Rosh Chodesh, which is a day of forgiveness and atonement.

Shulchan Aruch (YD 89:1): One may not eat cheese after meat until waiting six hours.

Rema: Our custom is to say Birkas ha'Mazon and wait one hour.

Yabi'a Omer (YD 1:4): Zivchei Tzedek says that even though we accepted the rulings of the Mechaber, if one is slightly sick, surely one may eat milk even after beef after waiting one hour, like the Rema. Chachmas Adam, Ben Ish Chai (2 Shelach 11) and others say similarly. Chelkas Yakov (3:147) is lenient to give to a child below nine years milk after meat. Children are no worse than one who is slightly sick. This is even for children above six. Aruch ha'Shulchan says so. Especially nowadays that weakness descended to the world to a terrible degree, one may be lenient for children.

Chayei Adam (Shabbos 139:1): Some have the custom to pray the entire Tefilah on Rosh Hashanah and Yom Kipur while bowed. However, after weakness descended to the world, it is better to pray with erect limbs and a bent heart and proper Kavanah.

Maharsham (5:30 DH ul'Dina): If a Kohen transgressed only once, even if he was Mezid, several Poskim permit him to give Birkas Kohanim after doing Teshuvah. Or Zaru'a says so about a Shali'ach Tzibur who murdered. It seems that the same applies here. Once the Kohen accepts to repent and bring 10 men to the grave to ask forgiveness, he is like a Tzadik. Noda bi'Yehudah (1: OC 35) says that the primary Teshuvah is regret, Viduy and a fence (to ensure that he not transgress again), and not the self-afflictions (to atone for his sin), especially nowadays that weakness descended to the world.

Avnei Nezer (YD 225:41): Even Akavya ben Mehalalel, who says that Yarok (yellow) blood is Tamei, a woman who finds a Yarok stain on her garment is Tahor. It is not common for Yarok blood to come from her body, so we do not attribute it to her. We can say that Yarok was red, for Yarok is uncommon. When there is some Raglayim l'Davar (circumstantial evidence) we attribute that this happened. However, nowadays we find Yarok more than red, because weakness descended to the world. If so, why should we assume that it was red, just because the ends are red? It is more reasonable to say that there was a mixture of (Yarok and) red.

Toras Chayim (Zonenfeld, 23 DH v'Lo): The Torah made the times for Keri'as Shma dependent on when people lie down and rose. It did not fix explicit times. If because weakness descended to the world, it is normal for people to sleep more or rise later, perhaps there is a Chiyuv mid'Oraisa (while some people are sleeping), especially if the climate causes (them to rise late).

Afarkasta d'Anya (2 OC 76 DH mi'Kol): There is a custom for the oldest of the house (where there is no Bechor) to fast on Erev Pesach. Orchos Yosher says that it is proper to annul this, for nowadays weakness descended to the world. Even if at the time it seems that he can fast, he will not be able to do the Seder at night properly. Also, Erev Pesach is (like) a Yom Tov (for we bring Korbanos on this day), and only Bechoros are permitted to fast. YHL"L (I do not know who this is - PF) says that even a male or female Bechor should redeem the fast if they are weak. Maharsham derives from the Shirei Korban that the Yerushalmi holds that Bechoros do not fast.