The Health Protection Agency (HPA) has found a potential link between 'ready-to-eat' salad bags and an outbreak of Cryptosporidium infection in May 2012.
The infection affected around 300 people in England and Wales for a short period and caused mild to moderate symptoms, but no deaths.
An investigation by the HPA found a "significant statistical association" between cases of the infection and consumption of pre-cut spinach, particularly from one major supermarket chain.
Another link was found with pre-cut mixed salad leaves from another major supermarket chain.
The HPA said members of the public do not need to take any action, but it hopes the findings will help prevent further outbreaks of this type from happening again.