A while back I started a thread Natural Swimming Pools (NSPs) where I analyzed the theoretical risk using the standards for such pools compared to EPA and DIN 19643 standards. The basic tradeoff for such pools is increasing the risk of ill swimmers from virtually nil to 7-10 per 1000 per season. However, as the links in Ben's earlier post in this thread show, even these looser biological standards aren't being consistently met, especially for Pseudomonas aeruginosa found in 27-28% of measured samples in 43% of pools and even with Legionella found in 3 out of 4 samples in an indoor pool. The mere presence of bacteria exceeding limits does not mean there will be associated disease, but the risk becomes higher.