In case you haven't seen this news article, evidently there is an increase recently of cases where people gets killed from a brain-eating amoeba in lakes. The discovery of said amoeba occurred in Australia in the sixties, and evidently it exists all around the world...usually in hot climates. The numbers of people affected are still tiny, of course...six people killed so far this year...but they do seem to be increasing slightly. Also, there's no effective treatment once you're infected, and usually, the cause of death isn't identified until after the fact.
All in all, I'm now glad I didn't go swimming in the lake at this one resort I visited earlier this month. I did go swimming once in the lake there a few years ago, and ended up getting a nasty sinus infection, but no deadly amoebas. At least, a well-maintained pool is safe, since the chlorine there kills all the bacteria (or amoebas) in the water.