The need for water increases with conditions of travel and recreation- exertion, hot weather, air travel, and altitude, to mention just a few. But finding safe water can be difficult. Sophisticated water purification requires proper disposal of sewage, optimum quantities of chlorine or other disinfectants, proper storing, filtration, clean and intact pipes, frequent testing, and continuous electric power. When power fails, for example, purification machinery stops operating. But the water keeps flowing.
Tap water is somewhat risky even in areas of good sanitation.
The US Public Health Service says that water containing up to 10 coliform organisms per liter of tap water is acceptable and unlikely to make anyone ill. In the wilderness, consider all natural water contaminated. Twenty years ago common wisdom was that clear, cool water upstream from large animals was potable. Shigella, salmonella, and hepatitis A organisms resist freezing. Giardia, the most common diarrhea-producing organism in the wilderness originates in small animals and birds.