Here's what you need to know:
What was recalled?
Amy's Kitchen, which makes organic products, recalled nearly 74,000 cases of them because of the listeria scare this week. For a full list of which products and what dates were on them, click here.
Three other companies -- Rising Moon Organics, and Superior Foods, Inc., and Twin City Foods, Inc. -- also recalled products because of contaminated spinach from an organic supplier. Twin City Foods said its products were sold at Wegmans Supermarkets, Inc., which also issued a separate recall because the spinach was sold under the Wegmans brand name.
Who supplied the greens?
Coastal Green said it notified the Food and Drug Administration as soon as it detected listeria during routine testing and realized some of its shipped product may have been contaminated, said spokesman Paul Fanelli. Coastal Green processes organic and conventional vegetables and is working with the FDA to resolve the listeria problem, he said.
"We're in the middle of an investigation here as to what the root cause was of the listeria," Fanelli said. "Once we determine what that is, we'll change our policies and our procedures accordingly."
Who got sick?
There have been no reported listeria illnesses tied to any of these products, but Wegmans and Twin City Foods said they issued recalls to be cautious.
Amy's Kitchen, Rising Moon Organics, and Superior Foods did not immediately respond to requests for comment.
What is listeria monocytogenes?
Listeria is a bacterium that lives in animals' digestive tracts but can cause an illness called listeriosis when consumed by humans. This happens when fruit and vegetable crops are contaminated by animal waste. That can happen because of tainted irrigation or wash water, or because animals got into the field.
What are the symptoms?
Laboratory tests can confirm diagnosis, and doctors will usually treat with antibiotics and fluids, he said.
How serious is listeriosis?
The deadly bacteria sickens about 1,600 people each year and kills about 260 people, according to the CDC. But healthy people who consume it don't always become ill.
Why is listeria problematic?
If food hasn't been heated thoroughly, listeria can live on even after its been cooked, Schaffner said. And unlike other bacteria, listeria can continue reproducing in cold temperatures such as a refrigerator and doesn't die in a freezer, he said.
"This is a rascal," he said. "It may create an infectious dose even though you've kept the food in the fridge."
What does the outbreak show us?
Food safety lawyer Bill Marler said the listeria outbreak illustrates how complex the food system has become, but that routine testing is effective.
"Products like frozen spinach travel all over the country and make it into multiple brands," he said. "It does make doing a recall a challenge, and if an outbreak [occurs, it can be] difficult to pinpoint the cause.
"On the plus side of the recall, it shows that testing of products [for harmful bacteria] works and being transparent with that information, as required by the FDA, will save lives."