Fact about eggs, & Salmonella: Only 1 in 1 million eggs will be infected with salmonella, contaminated egg cases are rare, but are somewhat more common in store bought eggs, since the hens kept at many egg farms are not treated with care, & kept in unsanitary conditions. They are in contact with mice, which do carry the Salmonella bacteria.
Anybody who owns chickens, & have mice running around are more likely to have Salmonella in there flocks. But mice aren't the number one cause. Its contaminated water, which is drank by the mice, & the mice poop everywhere the chickens eat, & drink, therefore infecting your flocks. Chickens aren't born with Salmonella, if the bacteria was in the egg while a chick was developing, the chick would die before ever hatching.