I heard recently that perhaps current CAFO beef feeding operations that are often blamed for salmonella poisoning, not because cows spread salmonella, but most because cornfed cows spread salmonella. The acidification of their stomachs from all that corn cause them to carry high amounts of pathogens that they would not normally have problems with. One of these pathogens is salmonella. Here is a very interesting interview detailing the life of a cornlot cow from birth until death: http://www.math.uic.edu/~takata/some_articles/FreshAir_Michael_Pollon_on_beef_industry,_hormones,_antibiotics.html
One of the things of note in this article is when they mention that one reason why corn is the feed of choice is because it is subsidized and so is much cheaper, even cheaper to buy than it actually cost to grow. So basically, your tax dollars help pay for the corn these cows eat.
The other thing of note is that the acidification and contamination of cow rumin also breeds acid loving strains of bacteria and disease. And since humans naturally have acidic stomachs as well, diseases bred in acidified cows are much more aggressive and successful in the human digestive tract than diseases bred in the natural neutral PH stomachs of naturally foraging cows.
SO basically, as the article describes, you end up with these sick cows digesting corn and all around them are giant open vats of feces contaminated with salmonella and all kinds of things. (and I can attest to the veracity of how bad these places smell as you can literally begin to smell the horrible stench from 10 or more miles away) But one thing I was told, and I am still looking for info on this, is that because the cornlot cows are fed corn, all the surrounding wild life that likes grain will sneak in at night and try to steal what corn they can get as well. That includes mice, rats, birds, rabbits, deer etc. All these wild animals track through the feces and consume the feces infested grain and because the feces are infected with so many diseases, many of the wild animals can take on these pathogens and also further spread them in THEIR feces. This is how salmonella can easily escape a corn lot and make it 5 miles away to contaminate a lettuce farm. It's not just water runoff that can do it. All the wildlife near a corn lot are infected carriers that infect the entire area. Anyone have any more info on this second bit? It sounds logical but I haven't yet been able to find any info on salmonella and other pathogens passing from cow feces to wildlife.