According to a recent study published in the European Journal of Epidemiology, around one in five sporadic cases of Legionnaires' disease could be the result of not adding windscreen wash to a vehicle's windscreen wiper water.
(The rare type of pneumonia is caused by water-dwelling Legionella bacteria. Not adding screen wash to the water may allow the bacteria to flourish under the bonnet of a car, which provides the perfect breeding ground. Having put together the last three IFs I'm beginning to get the feeling that something is out to get us.)