Avodah Zarah 12B

פרצופות למה לי? משום דקבעי למיתני: כיוצא בו לא יניח פיו על גבי הסילון וישתה, מפני הסכנה.

The Talmud asks: Why do I need the baraita to teach that it is prohibited to drink from fountains formed in the figure of human faces? If the reason is to teach the halakha in a life-threatening situation, the baraita already addressed this issue in the case of the spring. The Talmud answers: It was included because the baraita wanted to teach the continuation of that halakha: Similarly, one may not place his mouth on a pipe and drink, due to the danger that this poses.

מאי סכנה? עלוקה. תנו רבנן: לא ישתה אדם מים לא מן הנהרות ולא מן האגמים, לא בפיו ולא בידו אחת, ואם שתה ⁠— דמו בראשו, מפני הסכנה. מאי סכנה? סכנת עלוקה.

The Talmud inquires: What danger is the baraita referring to here? It is referring to the danger of swallowing a leech in the water. As A baraita states: A person should not drink water from rivers or from ponds either by drinking from the water directly with his mouth, or by collecting the water with one hand alone. And if he drank in this manner, his blood is upon his own head, due to the danger. The Talmud explains: What is this danger? It is the danger of swallowing a leech.

מסייע ליה לרבי חנינא, דאמר רבי חנינא: הבולע נימא של מים ⁠— מותר להחם לו חמין בשבת. ומעשה באחד שבלע נימא של מים, והתיר רבי נחמיה להחם לו חמין בשבת. אדהכי והכי? אמר רב הונא בריה דרב יהושע: ליגמע חלא.

This supports the opinion of R' Ḥanina, as R' Ḥanina says: In the case of one who swallows a water leech [nima], it is permitted to perform labor on Shabbat and heat water for him to drink on Shabbat, as his life is in danger. And in fact there was an incident involving one who swallowed a water leech, and R' Neḥemya permitted them to heat water for him on Shabbat. The Talmud asks: In the meantime, until the water is ready, what should he do? Rav Huna, son of Rav Yehoshua, said: He should swallow vinegar.