I'm sorry your daughter got ill, but, cows on grain diets are the likely cause of e. coli getting into your salad. One would think that cow manure would be a benefit to your local organic farmer. This would be true if cows, which are grass eaters, were only fed hay. Cows digest hay easily without creating acid-resistant e. coli. The human digestive system dispatches this form of e. coli handily. Cows do not digest grain easily, creating acid-resistant e.coli in the process. Run-off from grain-fed cattle farms into your salad grower's land is the likely culprit.
Source:
Science Daily from a news release by Cornell University