Makos 16b: Rav Bivi bar Abaye said: Someone who drinks from the vessel used by the doctor to draw blood is transgressing “lo seshaktzu”.
Shulchan Aruch Yoreh Deah 116:6: It is forbidden to consume food or drinks that are revolting to people, for example, if vomit, feces or puss got mixed into them. Similarly, it is forbidden to eat or drink out of dirty dishes, such as those used in the bathroom or glass containers used to let blood, and the like; and similarly one should not eat with dirty hands or on dirty dishes – because all of these are included in the prohibition “do not make yourselves disgusting.”
Shulchan Aruch Yoreh Deah 103:2 If the issur gives a bad taste to the food, but it is still edible, then as long as the issur is less than 50%, it is kosher.
מכות טז ע”ב: אמר רב ביבי בר אביי: האי מאן דשתי בקרנא דאומנא ־ קא עבר משום לא תשקצו.
שולחן ערוך יורה דעה קטז,ו: אסור לאכול מאכלים ומשקים שנפשו של אדם קצה בהם כגון משקים ואוכלים שנתערבו בהם קיא או צואה וליחה סרוחה וכיוצא בהם וכן אסור לאכול ולשתות בכלים הצואים שנפשו של אדם קצה בהם כגון כלים של בית הכסא וכלי זכוכית שמקיזים בהם וכיוצא בהם וכן לא יאכל בידים מזוהמות ועל גבי כלים מלוכלכים שכל אלו בכלל אל תשקצו את נפשותיכם.
יורה דעה קג,ב: פגם זה אין צריך שיפגום לגמרי עד שיהא קץ לאכלו אלא אפילו פוגם קצת אינו אוסר תערובתו ויש מי שאומר דהיינו דוקא כשנתערב איסור מועט עם היתר מרובה אבל איסור מרובה לתוך היתר מועט ואפילו מחצה על מחצה אין אומרים נותן טעם לפגם מותר עד שיפגום לגמרי שאינו ראוי למאכל אדם.
One Shabbos in the Ponevezh Yeshiva during the 1948 war, when the food supply was short, a pot of cholent sat cooking on the stove. One of the many hungry cats in the neighborhood smelled the cholent and wanted to eat some, so he climbed up onto the edge of the pot, stretched his neck down and tried to eat, but unfortunately, in the process he fell in and died. In the morning, the cook came in and saw that the volume of the cholent had increased. Eventually, he realized that a dead cat was in the cholent. They had a non-Jew remove the cat, but the question was whether the cholent was kosher or not.
There are two shailos here: 1) Is the cat considered “nosein taam lifgam”, so that its taste requires only a rov to be mevatel it – no need for 60 times? 2) Would this be considered “bal teshaktzu” – eating a disgusting thing? The cook asked the rosh yeshiva, who said that the cat was considered “lifgam” because in Eretz Yisroel, no one eats cats. Regarding the second question, he said not to tell any of the bochurim that a cat was cooked in the cholent, because when you don’t know, there is no prohibition of “bal teshaktzu”. By the time they found out, it was already half a year later. After that, they made sure to put a cover on the cholent every time.
Source: Kuntres Mah Nomar, Hilchos Kashrus, p. 139
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