This risk is vanishingly small. I did some quick calculations and I reckon that to save one life (on pretty questionable data) you would need to spend about £28M on windscreen washer fluid. Legionairre's disease is quite unpleasant, but usually curable, the deaths tend to occur in elderly otherwise unwell folk.
The government healthcare rationing outfit (NICE) reckon that spending more than £28-30,000 per life year saved is too much. I'd say that the risk is so small, and the cost is so great to avoid it that you should not decide to use washer fluid on that basis. I choose to use it because on the whole it works better than water, and doesn't cover your car in suds as detergent does.
As the septics say, YMMV.