The human mouth is like Palm Springs for bacteria. The warm, wet environment is the perfect breeding ground for germs, which is part of the reason why it's never a good idea to actually put your money where your mouth is (a 20/20 investigation found that paper money oftentimes contains germs of fecal, respiratory, and skin origin).

Mouth ulcers are a prime example of how quickly the mouth can become infected. More or less just an open wound, mouth ulcers most commonly appear on the inside of the bottom lip. Without treatment, an ulcer can hang around for anywhere from five to fourteen days. One way to help deal with the annoying pain typically associated with mouth ulcers is to apply choline salicylate gel (the active ingredient in many brand names, including Bonjela), according to a study published in the Journal of Canadian Dental Association.

One way to keep your mouth as bacteria-free as possible is to switch from that old school bottle of Listerine to newer kinds that do not contain alcohol. Research shows that alcohol-based mouthwash can actually increase the presence of bacteria in the mouth.