Let me ask you straight out: what do you use to fill up your windshield washer reservoir? If you use the blue stuff, the pink stuff or the green stuff (or any commercially available windshield washer fluid), you’re golden. Stop reading now, move along, there’s nothing to see here.
On the other hand, if you fill your windshield washer reservoir with tap water, you may want to use a respirator getting into and out of your car. It would also be a good idea to make sure you’re paid up on both your health insurance and your life insurance. You do have a will, right?
Why? According to BBC News, linked from Autoblog, drivers who fill their windshield washer reservoirs with water produce breeding tanks for Legionella bacterium, which causes (you guessed it) Legionnaire’s disease and pneumonia. Contaminated windshield washer tanks account for 20% of the Legionnaire’s disease cases in Great Britain, so it’s not like your odds of getting struck by lightening.
The good news is that you’re not likely to infect others around you, since they’re not exposed to your windshield-washer-biological-weapon for long. Professional drivers (cabbies, limo drivers, bus drivers) who tank up with tap water are at highest risk, so you don’t need to panic unless you spend a lot of time behind the wheel and have flu-like symptoms. Just buck up for the good stuff next time you buy gas and you’ll probably be fine.