I'm surprised at how quickly the natural pool concept was nuked off of this forum and although this post may be moderated and stifled as quickly I wanted to promote a little logic and perspective into the discussion.
I'd like to point out that humans have been swimming in ponds and lakes for all of 50,000 years up until the last century or two. Let that sink in. In all human existence were put out onto a year calendar all of our chlorinated pool swimming time on earth would be summed up on the last hour of the last day of December. So if we were taking baths before then (and I'm to understand we were and did a great deal) it seems like a natural pool wouldn't be that great of a health concern.
While I won't disagree that swimming in sterilizer is bound to keep you safe from any and all microbial life it is designed to destroy, I would like to point out that the same toxicity of any anti-microbial that doubles as an oxidizer does have it's downsides when exposed to the human body.
I'm sure when discussing disease in bodies of water the giant in the room is the amoeba Naegleria. I'd like to point out that it's found in almost every natural body of water you dig for it in and while millions are exposed per year from 2001-2010 only 32 were confirmed infected in the US and they didn't all die. However, an average of over 3500 DEATHs occur from drowning every year. That's 35 thousand people every ten years. Over a thousand times more people die from drowning than are infected with Naegleria.
I'd also like to know that at least 2 of those Naegleria deaths occurred after exposure received in a public chlorinated pool. While obviously the owner didn't clean and sanitize the pool properly it's fair to note that one likely cause could have been overuse of cyanuric acid which is known to inhibit the ability of chlorine to kill Naegleria.
How many people come to this forum because performing the chemical maintenance shuffle every year is fun, exciting, and easy and they don't often make mistakes? Judging by the name I'd assume it's safe to say you have a great many people who burn thousands of dollars to have a "trouble free" pool because they're actually a great deal of trouble.
I think given the relative facts. It's more than fair to say a properly maintained natural pool would certainly be more effective at preventing disease transmission than a lake or pond. And so would a chlorinated pool certainly. That being said the real issue would be, "Is a natural pool good enough while creating less work and a more desirable aquascape?"
Most of Europe would agree that it is. And given that people swim in lakes all the time in the southern states I'd have to say that given the right balance a natural pool would work pretty well in those climates too.
I'm not turned off by the discussion of chlorine. Surely it's an excellent disinfectant if used properly I wholeheartedly agree. But I do have a bit of a nag about blasting new ideas for the sake of repetition and consumerism. About an hour of googling turned the stats I referenced above. I hope for the interest of a "trouble free" pool you take another look at a not so new, but viable and interesting way to achieve that goal.