Given the environment, it can’t be guaranteed that you will not encounter some dangerous wildlife on your hike of the Appalachian Trail. However, you will typically only see black bears and they are typically shy. Then again, if they have cubs then stay far, far away from them.
As for snakes, you may also encounter them but most will be non-poisonous. To err on the side of caution, however, just assume that every snake you come across is venomous.