The Gemara describes the rabbis' fears of being poisoned. Some rabbis trust just about everyone. Others are wary of Gentiles, widows, and anyone else who might have left their food or drink in a place where a snake could find it. These comments lead me to believe that the bite of a snake was very dangerous and somewhat common.
Rav Safra teaches that there are three types of venom. A young snake's venom sinks; a middle-age snake goes to the middle. The venom of an old snake stays on the top of a liquid. Older snakes are understood as weaker than that of younger snakes. A baraita teaches that three thins get stronger with age: fish, snakes and pigs. But perhaps this indicates only that the venom of an older snake is weak while the snake itself is strong. The rabbis prove this by describing situations where the water at the bottom of a barrel is poisonous. They question whether or not we should permit a non-kosher animal, at cat for example, to drink from possibly poisoned water.