(Update: after posting this I found that actual experts have proposed a different hypothesis)
So the recent Legionnaires’ Disease Outbreak in Flint, Michigan has been getting quite a bit of press recently. For example in the last few days here are pieces from The Atlantic, The Washington Post, and CNN.
The very short version of the story is that over the last couple of years, Flint has seen a huge increase in the number of Legionella cases in the area. This correlates with a switch from using from Lake Huron water (via Detroit) to using water from the nearby Flint River. That switch clearly caused a number of issues due to iron and lead in the water… but it’s not clear if the Legionella outbreak is directly related.
Many of the news stories seem to mash together things like color and smell of the water (presumably unrelated to the risk of Legionella infection), the water quality issues, and the outbreak.
Since presumably neither Lake Huron nor the Flint River are host to significant number of the bacteria, there must be something else going on (unless the timing is a coincidence). It sounds like the water was corrosive, and that in turn is what released all the lead from the pipes in old buildings. Officials and experts are leaving their hypotheses fairly vague at this point but I’ll throw my personal hypothesis out there…
So Legionella has a well-deserved reputation for being very hardy… it can hang out in really tough biofilms and can also hide inside its natural host (amoebae). I’ll bet that many building water systems play host to Legionella biofilms at some low (undetectable?) level and in a way that doesn’t present much of a health risk. But if you radically change metal concentrations and send corrosive water through those pipes, perhaps it dislodges those biofilms… putting the bacteria into the water such that they can then be aerosolized in showers, air conditioners etc. and infect people.
(Major caveats: I only know what I read in popular news articles about this topic and I’m not an expert on Legionella)