Here's the straight poop on how salmonella gets on and into eggs. You may first want to put down anything you're eating.
Salmonella enteritidis, at the center of the outbreak, is a bacterium - a microscopic, rod-shaped, living creature - that can exist either within or on the surface of a shell egg. According to the U.S. Department of Agriculture's Food Safety and Inspection Service (FSIS) it can be transmitted to the outside of the egg as it travels through feces on the chicken's cloaca - the posterior chamber through which solid waste, urine and eggs pass.
However, the Centers for Disease Control and Prevention (CDC) notes that while this was historically the primary cause of eggborne contamination, "stringent procedures for cleaning and inspecting eggs were implemented in the 1970s and have made salmonellosis caused by external fecal contamination of egg shells extremely rare."
In the case of this massive contamination, though, the CDC reports that the problem is coming from inside the eggs. Laying hens can be infected either by contact with human workers who have not followed proper sanitary procedures, or, more frequently, by consuming feed that has come into contact with rodent feces. Affected hens can transmit the bacteria from their ovaries or oviducts before the shell even forms around an egg, thus making the egg's tainted status undetectable.
The CDC also notes that an infected hen won't always lay a bad egg. She can produce many normal eggs while only occasionally laying one contaminated with salmonella enteritidis - making spot checking of flocks a difficult proposition. The FSIS also notes that it has no information supporting the claim that chickens labeled "Kosher," "free-range," "organic," or "natural" have more or less salmonella bacteria than other poultry.
This doesn't mean that omelets are forever off the menu. Salmonella enteritidis can be killed by pasteurization or by heating the egg throughout to a temperature of 160°F, but if it is ingested without one of these treatments, it can affect the human intestinal tract.
The CDC reports that people in a normal state of health who ingest salmonella enteritidis-tainted food may experience diarrhea, fever and abdominal cramps, which typically begin within 12 to 72 hours. The Mayo Clinic notes that this may be accompanied by vomiting, chills, headache and muscle pains. These symptoms may last about four to seven days, and then go away without specific treatment.
Children, the elderly, and people with compromised immune symptoms should practice extreme egg caution, as the Food and Drug Administration reports that salmonellosis may lead to severe illness, arthritis, or even death.
Avoid bad eggs and crack open these safety tips from CNN Health.