חוששין שמא במקום נקב נקב
one is concerned that perhaps the bird or the mouse perforated it in the place of the preexisting perforation caused by a snake, and it is prohibited to eat the fig or the melon, due to the danger that the snake might have left its venom.
אמר ליה מי קא מדמית איסורא לסכנתא סכנה שאני א"ל רבא מאי שנא ספק סכנתא לחומרא ספק איסורא נמי לחומרא
Rav Huna said to Rabbi Abba: Are you comparing danger to prohibition? Danger is different, and one rules stringently in cases involving danger. Rava said to him: What is different about the fact that the ruling in cases of uncertainty involving danger is stringent, given that in cases of uncertainty involving prohibition the ruling is also stringent?
א"ל אביי ולא שאני בין איסורא לסכנתא והא אילו ספק טומאה ברה"ר ספיקו טהור ואילו ספק מים מגולין אסורין
Abaye said to Rava: And is there no difference between prohibition and danger? But isn’t it the halakha that in a case of uncertainty involving ritual impurity in the public domain, its uncertain impurity leaves it pure, while in a case of uncertainty involving water that is exposed and therefore susceptible to a snake leaving venom in it, the water is forbidden.
א"ל התם הלכתא גמירי לה מסוטה מה סוטה ברה"י אף טומאה ברה"י
Rava said to Abaye: There, in the case of ritual impurity in the public domain, the Sages learned this halakha through tradition from the case of a sota, a woman who enters into seclusion with a particular man after her husband warns her not to. She is forbidden to her husband even though there is uncertainty whether or not she committed adultery. Just as a sota is forbidden only in a case of uncertainty in the private domain, as there is no seclusion in the public domain; so too with regard to ritual impurity, one becomes ritually impure in a case of uncertainty only in the private domain.
מתיב רב שימי שרץ בפי חולדה וחולדה מהלכת על גבי ככרות של תרומה ספק נגע ספק לא נגע ספיקו טהור ואילו ספק מים מגולין אסורין
Rav Shimi raises an objection to the opinion of Rava from a mishna (Teharot 4:2): If the carcass of a creeping animal was in the mouth of a weasel, and that weasel was walking on loaves of teruma, and there is uncertainty whether the creeping animal touched the loaves and uncertainty whether it did not touch the loaves, its uncertain impurity leaves it pure, while in a case of uncertainty involving water that is exposed and therefore susceptible to a snake leaving venom in it, the water is forbidden.
התם נמי הלכתא גמירי לה מסוטה מה סוטה דבר שיש בה דעת לישאל אף הכא נמי דבר שיש בו דעת לישאל
The Gemara answers: There too, the halakha is derived from the case of a sota. Just as the uncertainty in the case of sota involves an entity that has consciousness in order for her to be asked whether she was unfaithful and is forbidden to her husband, so too here, only uncertainty involving an entity that has consciousness in order for it to be asked whether the loaves were rendered impure would become impure. The weasel does not have that consciousness.
אמר רב אשי ת"ש צלוחית שהניחה מגולה ובא ומצאה מכוסה טמאה שאני אומר אדם טמא נכנס לשם וכיסה
Rav Ashi said: Come and hear additional proof that danger is more severe than prohibition (see mishna Para 11:1): In the case of a flask of purification water that one left exposed and he came back and found it covered, it is ritually impure, as I say: An impure man entered into there and covered it, and in the course of doing so he rendered the vessel and its contents impure.
הניחה מכוסה ובא ומצאה מגולה אם יכולה חולדה לשתות ממנה או נחש לדברי רבן גמליאל או שירד בה טל בלילה פסולה
In a case where one left the vessel covered and came back and found it exposed, if it is in a place where a weasel could drink from it, or a snake according to the statement of Rabban Gamliel, or if there is concern that dew fell into it at night, the purification waters are disqualified for sprinkling in the process of purification of a person impure with impurity imparted by a corpse, due to the concern that the saliva of the weasel or the dew, which are unfit for sprinkling, intermingled with it. Nevertheless, the water is not impure.
ואמר רבי יהושע בן לוי מה טעם
And Rabbi Yehoshua ben Levi says: What is the reason that there is no concern that a ritually impure person exposed the waters and rendered them impure?