In a more casual (and not peer-reviewed) test, news station KLTV examined bacteria levels in water bottles that were used for a week without being washed. Bacterial cultures were taken from the necks of the bottles and the part that goes in a person's mouth. The results? "All of those grew lots and lots of bacteria that could make you very sick almost like having food poisoning," Richard Wallace, M.D., of the University of Texas Health Center, told KLTV. "That can cause nausea, vomiting, diarrhea. Basically the worst vomiting you have ever had in your life.