I found a very good article at this Web location:
http://www.jsonline.com/story/index.aspx?id=169860
Keeping the home hot water heater set very low could give rise to the growth of Legionnaire's Disease Bacteria (LDB). Of course, we've all been turning down the heat on our hot water heaters to save on energy bills.
I have also read that those shower heads that "atomize" the water present greater risk for your inhaling the bacteria. According to the article, some 80 percent of LDB infections come from sources other than large, institutional water heaters, plumbing systems or cooling towers. So that leaves . . . the home!
The "tankless" hot water heaters would seem to hold little risk of harboring LDB, and are also more economical to operate--although they cost much more than conventional hot water heaters.
This is one disease I do NOT want to get, as the mortality rates are quite high. In fact, the disease wasn't even really known until the 1970s. When no one had indoor plumbing, there wasn't much chance of LDB growing in your home. My grandparents' home in rural Alabama didn't have indoor plumbing--water was hauled from a well and boiled on a coal-burning stove. They did have electricity, which powered an ancient refrigerator and a radio usually tuned to barbershop quartets--but other than that, they basically lived like they had since the turn of the twentieth century.
I remember my mother and grandparents telling me how they lived during the Depression. People helped each other then--and I mean, they REALLY helped each other! I don't think we'd do that today if we had another disaster like the Depression. I wish I had evidence to the contrary.