Use of standard tap water insread of windshield washer fluid in the wiper reservoir, according to the BBC News, accounts for 20% of all Legionnaire's disease in the UK and can potentially cause pneumonia. Plain tap water, plus engine heat, caused the bacteria in the water to multiply and, when the wipers are used, sprays bacteria over the vehicle.
[Source: via autoblog | Image: williac – C.C. License 2.0 via autoblog]