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Since a few hundred million eggs have been recalled, I'm wondering how salmonella gets into eggs in the first place. Is it getting on the shells from the environment (or the hen)? Is it inside the egg?

This is mostly a curiosity/science question, so please don't take it as fear mongering, FUD, or any such non-sense. I realize that many eggs are perfectly safe.

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  • in addition to the (good) answers, it should be noted that the vast majority of such recalls are based on unfounded scares. People get infected with salmonella, turns out they had eggs a few days earlier, THEREFORE eggs are bad. Salmonella is found on a fried egg in the garbage, this is considered proof even though there's no way to know whether the contamination is primary or secondary (iow, the egg may well have gotten contaminated in the kitchen or garbage bag by something else).
    – jwenting
    Jun 10, 2011 at 5:59

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There are two basic mechanisms:

  • the salmonella is on the shell and when the egg gets broken the interior gets contaminated; or
  • the chicken's reproductive system is contaminated and lays an egg with contaminated interior.

To reduce the very common (a few percent) shell contamination, eggs sold commercially have their shells treated (typically washed). After washing only about 50 in every million eggs will still have some shell contamination. The shell contamination may have been there when the chicken laid the egg, from fecal matter, or as a result of plant processing.  

In some rare cases the surface salmonella can make it to the inside without breaking the egg: if it was there while the cuticle dried, if the egg is stored at room temperature, or if the environment is moist.   

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From LiveScience:

The bacteria, Salmonella enteritidis can invade an egg in several ways. One way is by the contamination of egg shells with fecal matter. The bacteria are present in the intestines and feces of infected humans and animals, including chickens, and can be passed to the eggs when chickens sit on them... Salmonella also silently infects the ovaries of healthy-looking hens, contaminating the eggs inside the chicken before the shells are even formed...

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    External contamination may be exacerbated by the hen sitting on the egg, but eggs and fecal matter both pass through the cloaca so if the chicken's feces carries Salmonella the shell may be contaminated as the egg is laid.
    – Dennis Williamson
    Aug 21, 2010 at 21:14

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