However, patrons may still be exposed to germs during the time it takes for chlorine to work or water to be recycled through filters. Most diarrheal outbreaks in pools appear to be related to fecal contamination of the water by someone who is ill with diarrhea. In addition, tiny amounts of fecal matter also rinse off all swimmers' bottoms as they swim through the water. Infectious diarrhea can contain hundreds of millions of germs in a single fecal accident.
Many pools use one filtration system for several pools.
This causes water from many pools to mix quickly, and distribute germs throughout connected pools in a very short amount of time. If other patrons then swallow the contaminated water, they may become infected and develop diarrhea or other illnesses. Since many illnesses can be spread by swallowing just a few germs, it is possible that a single diarrheal accident can contaminate water throughout the largest pool or waterpark. Skin infections are the most common infections spread through hot tubs and spas. The increased temperature of the water in hot tubs and spas can cause the chlorine to evaporate more rapidly than in swimming pools.