A "live tooth" has a blood supply. If a cavity extends into the "pulp" in the core of the tooth it exposes the tooth's blood supply to damage e.g. when brushing the teeth which can cause bleeding from the cavity. See a dentist for treatment. You may find it helpful to plug the cavity with a little cotton wool. If the cavity is causing pain, that can be relieved quite effectively by putting a drop of "Oil of Cloves" onto the cotton wool.
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I guess yeah, if your tooth cavity is deep enough.
yes cause cavity it hurts and rotten teeth (: