Candocook is right - the health of the animals involved really doesn't have bearing on the e-coli outbreaks.
Various strains of e-coli live in virtually all mammal feces - livestock, wild deer & pigs, & human feces included. Not all these strains cause illness, but obviously some do. Although I don't believe anything was absolutely positively pinpointed as the definite cause, the investigations with regards to the spinach outbreak tentatively traced the sometimes-fatal strain to possible water runoff from a neighboring cattle farm, even though there are apparently regulations in place as to legal distances to be kept between livestock & produce production.