How does Salmonella contaminate eggs after they are laid?

While it is a common misconception that all eggs are contaminated with Salmonella when laid, more than 90 percent of eggs are actually free of the bacteria at that time.1

Contamination with Salmonella typically occurs after the egg has been laid. This can happen when the shell is exposed to infected fecal matter during storage or handling. To mitigate this risk, commercial processing involves washing and sanitizing eggs, which effectively removes most bacteria deposited on the shell surface.21

In rare instances—occurring in fewer than 1 percent of all eggs—the bacteria Salmonella enteritidis can be transferred directly through the hen's ovary before the shell is even formed. Because this internal contamination can occur, it is essential to prioritize food safety by cooking eggs thoroughly until both the whites and the yolk are firm, as this process destroys the bacteria and prevents foodborne illness.13