What was the cause of the Taco Bell poisonings? Do we even know, for sure?

Last answer I heard on the news was contamination of lettuce before it ever left the farms. E-coli can't just be washed off of lettuce or leaf spinach because it "adheres" to it. The CDC ( Center for Disease Control" WHO ( World Health Org ) & Wikipedia all have great info. If you don't trust something don't eat it.