How does salmonella get into eggs?

Don’t assume that an egg has to be cracked to contain salmonella. Some chickens carry salmonella in their ovaries, so the bacteria is in the egg when it is laid, explains food scientist Susan Brewer.  Consumer Reports On Health points out that salmonella bacteria can also penetrate the porous shell of an egg, and bacterial growth in an infected egg is especially likely to occur if eggs are not refrigerated very soon after being collected.
 
Source(s):
Susan Brewer, Ph.D., University of Illinois, Department of Food Science and Human Nutrition
 
Consumer Reports On Health, October 2009.