The water system most likely had not been giardia-contaminated for 50 years. Water systems and water sources in the U.S. have become contaminated...the vast majority did not start that way.

Until the 50s or 60s back-country water sources were pretty safe, especially high-country sources. We backpackers, with help from livestock and grazing permits, changed that. Missoula, MT had some nice uncontaminated mountain water until sometime in the 60s but that changed and hikers and their dogs got the blame. Happily, uncontaminated sources remain throughout the country and there are studies that suggest giardia is not as widespread as feared.

Clean systems sometimes become contaminated but can be returned to a safe condition. One of the reasons for the growing popularity of UV treatment of municipal water is it's better efficacy against giardia when compared to the more traditional chlorine treatment.

FB