An earlier investigation hinted at hospital food as a source, but evidence didn't reveal a specific item.
FDA and USDA teams are still working to identify a food source for the Listeria outbreak.
Illnesses have been occurring since 2018, mainly in nursing home residents, with the latest cases reported this year.
Of the 13 outbreaks with confirmed cases of foodborne illness, all but 1 involved either Listeria, Salmonella, or Escherichia coli.
The outbreak occurred last summer and fall and sickened at least 61 people from 19 states.
Following Listeria outbreaks and recalls, an ongoing review identified several steps that can be quickly implemented.
Seven cases have been related to pregnancy.
In California, one pregnant woman and her twins were sick, and both infants died.
The latest report on the 19-state outbreak noted 2 more cases, 61 total.
The researchers found foodborne pathogens that had formed colonies of pathogens in a sticky biofilm in three common types of plastic in raw and treated wastewater.
