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. Another opinion in the baraita limits the prohibition to cases where the person has an open wound, but if the person has no open wounds this would not be a concern.
The care that is required by the Gemara to avoid coming into physical contact with a snake’s venom actually depends on the type of snake. Most of the poisonous snakes in Israel and the surrounding areas are vipers, whose venom is made up of enzymes. Such venom could not affect a person simply by physical contact, unless there was an open wound that would allow the venom to enter the bloodstream. Other types of snakes have venom that affects the nervous system, which could enter the body through the eyes, for example, which is why washing the face with water that contains venom may be dangerous.
Given that the concern with uncovered food and drink is related to a very specific issue, the Gra (Shulhan Arukh Yoreh De’ah 116:1) accepts the argument made by Tosafot that these rules apply only in times and places where such snakes are found, but the Sages never intended to extend the prohibition to other places where they are not found.