Question:
Is it possible, I wonder, for a human to build up sufficient resistance to giardia that it is not a problem? (not that I'm anxious to try it) If indigenous populations live well enough without dying from the water, it seems likely. I wonder what the circumstances of Native Americans with respect to this, prior to the influx of Europeans were. Does anyone know all of the waterborne microbes which present health problems?

Answer: I dunno, could be, I was drinking the water, granted it usually was in the form of espresso, and did have a separate supply I trucked in, but did not use it, saving it for a couple of small children I was caring for who came into my care with extreme dose of the "hershies," etc. I did not get sick ... Chlorination is used extensively by municipal water treatment plants to disinfect water. However, the gaseous chlorine used by these plants is much too dangerous for home use. Household bleach (a 5.25% solution of sodium hypochlorite which is equivalent to 5% available chlorine) can be used for disinfecting drinking water (How to Super Chlorinate). When chlorine is fed into water, it first reacts with any iron, manganese, or hydrogen sulfide that may be in the water. If any residual (un-reacted) chlorine remains it will next react with any organic material (including bacteria) present. In order to ensure that the water remains protected throughout the distribution system, an excess of chlorine, usually .5 parts per million (ppm) is added. In large systems chlorine will be added again at distribution junctions. This "rate of feed" is normally adjusted to make sure that sufficient chlorine is available to fully react with the organics present. When both the mineral and organic reactions have been completed, any residual chlorine remains in the drinking water. Most people find the taste of water with residual chlorine to be objectionable but they do get used to it! Chlorination kills many pathogenic bacteria (including those which cause typhoid, cholera and dysentery), however cyst forming protozoa (Cryptosporidium) which cause amoebic dysentery, and giardiasis are extremely resistant to chlorination.