[MrB]

I was pretty sure that windshield washing fluid contains a significant quantity of alcohol, and bacteria don’t tend to survive in alcohol...

[trisham]

I found this, which doesn’t mention alcohol..

http://www.ehow.com/facts_5813733_windshield-washer-fluid-made-with_.html

Could it be that only those formulas that don’t contain alcohol are the problem?

[gtk]

Some bacteria don’t seem to have a problem with alcohol.

http://www.nbcnews.com/id/42240430/ns/health-infectious_diseases/t/two-thirds-alcohol-wipes-test-contaminated-bacteria/#.U3vLySgkR8E