By Debra Goldschmidt CNN

(CNN) -- Listeria contamination has prompted a flurry of food recalls in the past three weeks.

Fourteen voluntary recalls have been announced related to a listeria outbreak that has caused eight cases of illness in three states. On April 23, CRF Frozen Foods recalled 11 of its products because of possible listeria contamination. Then, on May 2, it expanded that recall to include all frozen vegetable and fruit products that had been processed at its Pasco, Washington, facility since May 2014. This accounts for 358 products sold under 42 brands.

Since then, at least 13 additional recalls have been announced that are linked to the CRF recall. Most of them are frozen vegetable products or contain ingredients that were part of the CRF Frozen Foods recall. The products were sold throughout the U.S. and in Canada.

The illnesses actually date to September 2013, according to the Centers for Disease Control and Prevention. All eight individuals have been hospitalized, and there have been two deaths, although listeria is not the cause of death in those cases.

Separately, on May 3, SunOpta announced a voluntary recall of sunflower kernel products produced in February at its Crookston, Minnesota, facility. Since then, 17 voluntary recalls have been announced by companies that use sunflower kernels supplied by SunOpta in their products, which have been distributed throughout the U.S. These include salads and trail mixes.

No illnesses have been reported linked to the potentially contaminated sunflower kernels.

"Sometimes, it takes time to identify where all the implicated products have gone," said Michael Doyle, director of food safety at the University of Georgia.

Symptoms of listeriosis infection include fever, muscle aches and gastrointestinal illness, such as diarrhea. It can also lead to meningitis. Pregnant women, newborns, older adults and anyone with a weakened immune system is most at risk for infection. Cervical infections caused by listeriosis in pregnant women may result in stillbirth or spontaneous abortion during the second or third trimesters.

The CDC estimates that 1,600 people a year come down with a listeria illness, and 260 cases are fatal.

Dr. Robert Tauxe, director of the CDC's Division of Foodborne, Waterborne and Environmental Diseases, says there is probably one more case for every case they know of. He also notes that listeria can be particularly challenging for food manufacturers to eliminate because it can hide in places like drains or pipes for years.

But the presence of listeria in products is going down, according to Doyle. And "not all strains of listeria are equally harmful to people," he said.

He is more concerned about the frozen vegetable recalls because they were prompted by an outbreak, so it's known that the strain is making people sick.

Though the sunflower seeds and products that contain them are not associated with an outbreak, if that strain were to be making people sick, Doyle said, he'd be more concerned: Unlike vegetables that are heated and cooked, sunflower seeds usually are not. Cooking and heating can kill the bacteria and reduce the risk of infection.

Investigations are underway to identify the source of the listeria contamination for both the vegetables and the sunflower kernels.

For a complete list of recalled items, check the FDA website. Recalled products should not be consumed or sold.

TM & © 2016 Cable News Network, Inc., a Time Warner Company. All rights reserved.