For the source text click/tap here: Avodah Zarah 30
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A number of different concerns are presented by the Gemara regarding food and drink that have been left uncovered. The main issue appears to be the concern that a snake may come to eat or drink from the uncovered liquid and leave behind venom that may injure or even kill.
The Sages forbid drinking from a barrel that had been left open even if others have already drunk from it, arguing that the venom may be at the bottom of the barrel. Furthermore, the Gemara quotes a baraita that forbids using water that had been left uncovered overnight even to wash the floor, to give to animals to drink or to use to wash his hands and his face.