Hippocrates recommended that water be boiled and strained to purify it. People in the middle ages boiled, filtered, and even distilled water to purify it. The

Hippocrates recommended that water be boiled and strained to purify it. People in the middle ages boiled, filtered, and even distilled water to purify it. The catch is that without germ theory, people mostly did this process to water which was obviously contaminated. If water looked clean and tasted clean, why purify it? Even people who should know germ theory still make this mistake—people who go backpacking will take water from a clear stream and drink it, unaware that there is a beaver dam upstream and giardia in the water.