Investigators from the Food and Drug Administration are carrying out tests at two large egg producers in Iowa. They think the two enterprises, Wright County Eggs and Hillandale Farms of Iowa, supplied eggs that carried salmonella bacteria and made thousands of people sick this summer.
More than 500 million eggs from those farms have now been recalled, and investigators are piecing together how the outbreak spread.
So this week, millions of Americans are opening their refrigerators, pulling out a carton of eggs and looking for information that will indicate whether theirs are safe to eat. On the cartons, they might find a code like 225 P 1979.
But just what does that code mean?
The first three numbers are the ordinal date -- so for Jan. 1, it would be 001, and Dec. 31 would be 365. That date indicates the date the eggs were packed. And the information after the date?
"That's a code that tells you which actual processing plant processed those eggs, and each plant has an individual number," says Pat Curtis, the director of the National Egg Processing center at Auburn University.
You can learn which numbers to look for, and which eggs to throw out, at the website foodsafety.gov.
These numbers also helped scientists identify the farms that now are under investigation.
The outbreak began last May. By August, at least 1,000 more people than usual, all around the country, had gotten very sick with salmonella poisoning. But no one knew what was causing it.
Hospitals reported the cases to state health authorities, who took a kind of genetic fingerprint of the bacteria and passed that information along to the federal Centers for Disease Control and Prevention in Atlanta.
Ian Williams, who is in charge of the outbreak response and prevention branch at the CDC, says in this case, the genetic fingerprint wasn't very helpful. It was a very common strain of the salmonella enteritidis bacterium, so investigators couldn't tell if all those people really were getting sick from the same thing.
"So one of the approaches we did to try to address this was to identify clusters of cases who had eaten at a common restaurant or event in a similar time period," Williams said. "You all went to a prom together, or a church picnic -- something like that."
Investigators then would search for the common link: What did all of these people eat? And did people in the other clusters eat the same thing?
"And the real breakthrough came toward the end of July when our colleagues in California had actually identified six of these clusters and they asked the simple question of 'We know there was salmonella enteritidis, and common sources are either chicken or eggs,' " Williams said. "And they simply asked the question, 'I wonder where these six clusters are getting their eggs from.' "
The egg cartons carried dozens of brand names -- from Albertson to Wholesome Farms -- but the plant numbers on the side of those cartons told the real story. Most of the people in the clusters had eaten eggs from just three different packing plants operated by two companies in Iowa.
FDA investigators now are trying to confirm the presence of salmonella at those farms. Often, it comes from mice that get into chicken feed and leave droppings, which then infect the chickens, who in turn pass it on to the eggs.
The Iowa facilities are huge -- each one of them has more than 1 million chickens. Some critics of the food industry say that such mammoth industrial operations are giving us more contaminated eggs. But Hongwei Xin, director of the Egg Industry Center at Iowa State University, says scientists haven't found that to be true.
"Whether you have a flock of 100 birds or 100,00 birds, if mice get into your system you'll be equally vulnerable," Xin said.
But if your chicken house is huge, any problem is really big, too. FDA officials say they can't be sure that this egg recall is the biggest one in history, but they can't remember a bigger one.