27. Germs-Offering someone to drink from one’s cup and drinking from another person’s cup:[1] [Shulchan Aruch/Achronim]

One should not offer his friend to drink from a cup from which he drank from due to danger of life.[2] [Likewise, one should not drink from a cup that another person drank from, even if he was not offered to drink from the cup.[3] Thus one is not to drink leftover wine or water from the cup of another, even if he is one’s brother or other relative.[4] However, some are lenient in this matter.[5] This leniency especially applies by Kiddush.[6] Nonetheless, one is to clean the cup prior to offering to another as explained next.[7]]

If one cleans the cup:[8] If one nonetheless desires to offer his drink to someone [or to take another person’s cup[9]], then he is to clean the cup through either pouring out some water from the side of the cup, or by cleaning the area of the lip of the cup from where he drank, prior to offering another person to drink from it.[10] [Nonetheless, if one is not aware if the drinker is in good health, then he should not drink from his cup even after it is cleaned.[11]]

[1] Michaber 170:16; Derech Eretz Raba 9; Tzavaas Rebbe Eliezer Hagadol 16; Kaf Hachaim O.C. 170:59; Y.D. 116:97; See Piskeiy Teshuvos 170:20; Sefer Shemiras Haguf Vihanefesh [Lerner] 42

[2] The reason: As one’s friend may feel compelled to drink from it due to embarrassment, and will end up falling ill and die due to being repulsed from having drunk from his friends cup. [M”A 170:21; Taz 170:8 in name of Bach 170 in name of Rashal] Alternatively, the reason is because once friend may have an illness from which one can die from. [Taz 170:8 in name of Rebbe Eliezer Hagadol 16]

[3] Taz 170:8 in name of Tzavaas Rebbe Eliezer Hagadol ibid and that so is the witnessed custom of people to be very careful not to drink from the cup of another; Peri Toar Y.D. 116:6

The reason: The reason for this is to prevent catching any disease or bad germs from him. [Taz ibid]

Other opinions: Some Poskim rule that the entire issue is simply with being offered a drink from another however one may decide on his own to drink from the cup if he desires. [Taz 170:8 in name of Bach 170 in name of Rashal; Implication of M”A ibid]

[4] Peri Toar 116:6

[5] Shulchan Hatahor 170:16

[6] Zivcheiy Tzedek 2:27

[7] Piskeiy Teshuvos ibid footnote 96

[8] Michaber 170:22; Tur 170:16; Tamid 27b; Tashbeitz 286; Sefer Chassidim 111; Kneses Hagedola 116:38; Aruch Hashulchan 170:19; Kaf Hachaim Y.D. 116:97; Darkei Teshuvah 116:99; See Hamelech Bemisibo Vol. 1 p. 269-270 that a) this is the correct thing to do [Reply of Rebbe to Rav Yaakov Katz, in opposition to Rav Zalman Duchman] and b) That this is a totally separate matter from the above.

[9] M”B 170:37

[10] The reason: This is done to clean the cup from any possible germs that the person being offered may be opposed to consuming. This is done either due to the germs being repulsive [Michaber ibid] or due to it spreading disease. [Taz 170:8 in name of Rebbe Eliezer Hagadol]

[11] M”B 170:37 based on Taz 170:8 in name of Rebbe Eliezer Hagadol; See Shaar Hatziyon 170:30; Kaf Hachaim 170:76