After coming back from a trip to India, an old housemate of mine told me that a huge part of the world uses their hand to wipe, and he decided to adopt the practice himself. He then warned me not to drink out of a cup he'd left on the floor in the bathroom.

I'm also reminded of a story I'd heard about an American traveling on a train in the Middle East, who'd offered to share some tomatoes he'd brought with him and divided up at lunch, when everyone else in his train compartment was sharing their food. All the other people were happy to share their own food, but none would touch the tomato sliced he offered them. He couldn't figure out why, but in retrospect he realized that it was because he'd touched them with his left hand, which in that part of the world was commonly used to wipe. So everyone would only shake hands with and touch food with their right hand.

I'm not sure how common such practices still are in the modern world, but if paper and bidets wind up in short supply, they're something to think about.