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: There were only two possibilities which may have caused me to contract Shigellosis at NC IMO, one, having taken a swim in the creek, below the lowest camp on the 4th (got sick on the 5th) (although I think Shigellosis is spread through fecal contamination, it could maybe have gotten into the water a number of ways) , or possibly through food contamination. I do not know which, or if either, was the actual source.

Chlorine as stated above is a very effective disinfectant and has been used in drinking water supplies for nearly 100 years. What concerns health officials are the chlorination by-products, "chlorinated hydrocarbons,"known as trihalomethanes (THM's). Most THM's are formed in drinking water when chlorine reacts with naturally occurring substances such as decomposing plant and animal materials. Risks for certain types of cancer are now being correlated to the use of chlorinated drinking water. Suspected carcinogens make the human body more vulnerable through repeated ingestion and research indicates the incidents of cancer are 44% higher among those using chlorinated water.