There is no scientific evidence to support the claim that people who regularly drink contaminated water build up an immunity to harmful bacteria. Even if we did build up an immunity to one particular type of contaminant, for example E.coli, that would not necessarily mean we would be immune to the harmful effects of drinking water containing other types of contaminant, for example cryptosporidium. Even if you believe you have built up an immunity to some of the bacteria in the water, other people visiting your property may be at risk, especially the young, the aged or people who do not enjoy good health.