Apparently
windscreen washer water is a likely cause of Legionnaire's Disease.
Researchers claim that "professional drivers are five times more likely to be infected." - but did they control for whether the cars in question had air conditioning or not? Given the stagnant smell that air-con can produce on a hot day, I'd be more worried about what was being living in the AC vents than what was living in the screenwash bottle (especially if as they recommend, you use a decent screenwash, they are mostly isopropyl alcohol or just ethanol).