14 people have tested positive at an Evesham food packing firm and all workers at the site are now being tested as a precautionary measure.
Public Health England and Worcestershire County Council are working with the company involved and monitoring the situation closely.
About 340 workers are employed at the site in Blackminster, which packs and distributes vegetables and salad products.
All staff will now be tested and advised to isolate when not in work.
PHE said it had seen a "trend" of outbreaks at large food producers and testing all staff was a "precautionary measure".
The firm said a host of preventative measures were in place, including screens, hand washing facilities, extra cleaning and protocols for workers coming and going on the site.
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