From NPR News, this is All Things Considered. I'm Michele Norris. Today, three more companies recalled products containing peanuts because of concerns about salmonella food poisoning. More than 125 companies have recalled peanut products this month.

And since the outbreak began last September, nearly 500 people have been infected with salmonella. The outbreak sounds large, but these cases are only a tiny proportion of the salmonella infections in the U.S. each year. NPR's Joanne Silberner explains what makes this outbreak special.

JOANNE SILBERNER: When you get infected by salmonella, when you suffer the nausea, vomiting, diarrhea, and fever - you know you're sick. Maybe a food preparer didn't wash his hands, or maybe you used the same cutting board for raw chicken and vegetables. Or maybe you ate some widely available product that contained salmonella.

The investigation starts at the Centers for Disease Control and Prevention where Dr. Ian Williams heads the outbreak team.

Dr. IAN WILLIAMS (Chief, OutbreakNet, Centers for Disease Control and Prevention): When people get ill, they go to their doctor, and they get diagnosed. And the samples that are taken from those people with diarrheal illnesses are forwarded on to the state public health laboratories that further characterize them and actually do a genetic fingerprint of the bacteria that made them sick.

SILBERNER: There are more than 2,000 types of salmonella. When the CDC sees the same bacterium from people in different states, it gets moving.

Dr. WILLIAMS: Our job is to figure out what do all these people have in common, and then try to see, if we can figure that out, how we actually stop the outbreak from happening.

SILBERNER: There are an enormous number of options to consider, says William Schaffner. He's an infectious disease expert at Vanderbilt University. Salmonella, he says, can come from anywhere.

Dr. WILLIAM SCHAFFNER (Professor and Chairman, Department of Preventative Medicine, Vanderbilt University): It's in animals, and so, being in animals, it can get in meat that we consume, but we've increasingly been aware that it's in fruits and vegetables.

SILBERNER: And it can spread through contaminated water. No one is sure yet how the salmonella got into the peanut butter and peanut paste. The current investigation started last fall when two related types of salmonella bacteria showed up across the country. State and federal officials asked hundreds of patients what they ate.

Many of the people were in nursing homes or had eaten at school cafeterias or other places that used large containers of peanut butter. A joint effort by the CDC, Food and Drug Administration and state officials traced the bulk peanut butter to a plant in Blakely, Georgia. Again, the CDC's Ian Williams.

Dr. WILLIAMS: We started to trace it backwards, to sort of say, where did this come from? And as we understood the distribution chain, we then realized the company that produced this peanut butter also produced a number of other peanut butter and peanut paste which went in to a number of products. So, we traced it back and then started to look forward.

SILBERNER: And that explained how people who had not eaten in institutional settings, but had eaten peanut butter crackers or cookies or cake or ice cream had gotten sick. The FDA began warning about the outbreak earlier this month. And nearly every day, more products show up on its recall lists.

The outbreak isn't over. Reports of new infections are still coming in. Meanwhile, the government's advice remains the same - don't worry about the peanut butter you buy from retail stores, but avoid products like crackers, cakes, cookies, ice cream, and even dog biscuits that contain peanut butter or peanut paste, unless you know they weren't made with peanut products from that Blakely, Georgia plant. Joanne Silberner, NPR News.

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