How do eggs get infected with salmonella?

From the chickens: Chickens are common carriers of salmonella in their stools and the disease can pass transovarily from an infected chicken to the outside of her egg shell, even if the egg is otherwise intact and healthy.

Chickens: An egg can become contaminated from salmonella bacteria in the chicken’s reproductive tract before a shell even forms. The same passageway that excretes feces is the one that excretes eggs – so the outer shell can be contaminated. Bacteria can also enter the pores of an eggshell after the egg is laid.