The FDA says an onsite investigation at Prairie Farms Dairy revealed three environmental swabs taken in the processing area tested positive for listeria monocytogenes.

Several cases of a popular nutrition shake have been recalled due to listeria contamination. The FDA and CDC, in collaboration with state and local partners, are investigating illnesses in a multistate outbreak of infections linked to Lyons ReadyCare and Sysco Imperial Frozen Supplemental Shakes manufactured by the Indiana-based Prairie Farms Dairy.

The products were distributed to hospital patients and long-term care residents by foodservice business Lyons Magnus.

The U.S. Centers for Disease Control and Prevention has been trying to find out the cause of these outbreaks since 2018.

Here is what you need to know about the deadly listeria outbreak, including which products are being recalled.

A listeria outbreak tied to supplement shakes is now linked to at least 12 deaths and 38 cases across 21 states, according to the Centers for Disease Control and Prevention (CDC).