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: Cryptosporidium is present in from 65 to 87 % of surface water samples tested throughout the United Sates. Although this protozoan is rarely in sufficient quantities in drinking water to cause problems, it is highly resistant to chlorination, and disinfection of water is not a reliable method for preventing exposure to it. In 1993 water plant operators in the District of Columbia began having difficulties maintaining optimal filter effectiveness. It was found that the water contained unacceptable levels of the parasite C. parvum, despite the fact that the water was adequately chlorinated. A telephone survey of 0.2% of the 600,000 residents of that area showed that 3.5% reported having diarrhea shortly after this episode.

The early detection of water-borne outbreaks of cryptosporidiosis is difficult because 1) many physicians are unaware that it can cause watery diarrhea; 2) the symptom complex often resembles a viral syndrome; 3) clinical laboratories often do not routinely test for Cryptosporidium when a stool examination for ova and parasites is requested; and 4) few states include cryptosporidiosis as a reportable disease. (Current Problems in Pediatrics, March 1995)