That's the problem with the "illegal pooping in the fields" canard: the scale of the outbreak indicates a systemic problem somewhere along the chain. It would be more accurate to claim that Mexicans are pooping into our distribution equipment. (Just that the image of a Mexican pooping into a piece of equipment doesn't garner the same emotional reaction).
All of these fields are artificially irrigated. One need only contaminate the irrigation source. A much more likely source for Salmonella than human is contamination with animal waste. Swine and poultry are the most likely sources.