We've all enforced it. Even if we shouldn't have, we did. The five-second rule. If food falls on the floor and you pick it up within fives seconds, it magically repels are bacteria it would have picked up during second six.
But is this really something we should be using?
Shockingly, no. However, scientists have been able to prove that certain foods will attract less bacteria in the same five second span.
Foods with a high moisture content (cheese, fruits, etc) attract way more bacteria than foods that aren't. Bacteria LOVE water, and will find it's way there as fast as possible. That's why damp clothes and wet sponges can start to smell so bad: they're filled with bacteria. It's probably for the best that you throw away moist foods if they end up on your floor. Even if you just cleaned it, those buggers find a way.
The other factor we need to take into account is what type of surface the food has fallen onto. Carpet is the cleanest surface to eat off (though it may be the fuzziest.) Tile and stainless steel were the nastiest. Carpet, because of it's uneven surface, limits how much of the food is in direct contact with the surface, minimizing the amount of bacteria it touches.
All together, it's probably not a great idea to eat food off the floor. But it's not going to kill you unless the floor is covered in poison. Bacteria is all around us everyday. Just touching your phone or your keyboard to read this article has got you covered in it. It's all about using your best judgement. Piece of watermelon on the floor? Probably not a good idea. A Dorito? Worth the risk.