“If you use standard tap water in your windshield washer fluid reservoir instead of a cleaner, you may have effectively turned your vehicle into a biological weapon. Sure, that sounds cool and all, but according to BBC News, the only person you’re going to be hurting is yourself. As it turns out, using plain water can cause the washer fluid system to become a breeding ground for Legionella bacterium – the same nastiness that causes Legionaires’ Disease and pneumonia. Spray your windshield and the bacteria becomes airborne, allowing it to easily enter your lungs and wreak havoc with your immune system.
Researchers discovered the hive of scum and villainy lurking under the hood by attempting to discern why professional drivers were five times more likely to become ill than their amateur counterparts. After a little scientific sleuthing, the lab coats unearthed the bacteria. So do the world a favor and top off your windshield washer fluid reservoir with some sort of purpose-built cleaner. The stuff will kill the infection-causing bacteria and will keep the fluid from freezing in the winter. Not bad for 99 cents a gallon.” (Autoblog).