The folklore that any water a dog will drink is pure enough for his master is false. The same notion for a horse is also false. Smell or lack of it is no guarantee of water purity. Sulfurous water smells horribly, yet it is safe to drink boiled or when not polluted. The fact that native people may assert a water source is pure could indicate they have built up a degree of immunity. To their systems, the water is not tainted (polluted). Even the loneliest wild stream can be infected with so called rabbit fever by wild animals such as muskrats and beavers.
Water can be rid of germs by boiling. The exact time required to accomplish this depends on altitude, the nature of impurity and several other factors. A safe general rule is at least 10 minutes, longer is by no means a waste except for fuel.
Sickness from consuming bad water can last weeks and sometimes results in death. This applies to water actually consumed and with equal gravity, to any water entering the human body. For example, water in which a toothbrush is dipped, water in which food and utensils are washed, and water used in cooking (except when kept at high enough temperatures for a sufficient time to insure purity) can all be sources for disease.