Rodent feces are the usual source

Test results from environmental samples at Wright County Egg - one of two related companies believed to have supplied Salmonella - contaminated eggs now under nationwide recall - showed strains of the bacteria with the same DNA fingerprint seen in clinical isolates. Chicken feed at facilities supplying Iowa-based Wright County Egg and Hillandale Farms also tested positive for Salmonella enteritidis with the same genetic fingerprint as 1,300 ill people across the U.S.

In the legal context, any possible defenses that Wright County Egg might have been able to raise are "up in smoke."