An outbreak of Cryptosporidium that affected around 300 people probably originated in ready-to-eat bagged salads, an investigation has found.
The Health Protection Agency (HPA) said there was "strong evidence" of an association between those who fell ill in England and Scotland and the salads, which were likely to have been labelled ready-to eat.
A sample of 25% of those who became unwell found 46% recalled eating mixed leaf bagged salad from the Morrisons supermarket chain - an "extremely high" rate as not everyone could remember which particular products they bought, while 11% ate spinach from the Asda chain, the HPA said.
The outbreak was short-lived, with most of those affected suffering a mild to moderate form of the illness. There were no deaths. The Cryptosporidium parasite causes the disease cryptosporidiosis, of which the most common symptom is diarrhoea. Potential sources include contaminated water or food and swimming in contaminated water.
The Food Standards Agency (FSA) said its own investigation into the outbreak was inconclusive as to where the point of contamination occurred. But it said consumers could still have confidence in the ready-to-eat label. It added that bagged salads sold in supermarkets were often sourced from the same suppliers for most leaf types, often with common production lines packing products for several retailers at the same time, which was the situation in this case.
The HPA's head of the multi-agency Outbreak Control Team, Dr Stephen Morton, said: "This outbreak was fortunately short-lived but it was important to see if we could find the source. Our findings suggest that eating mixed leaf bagged salad was the most likely cause of illness. It is however often difficult to identify the source of short lived outbreaks of this type as by the time that the outbreak can be investigated, the affected food and much of the microbiological evidence may no longer be available.
"As this was an isolated and short lived outbreak there is no specific action for the public to take but we hope the investigations between the FSA and the food industry will help to prevent further outbreaks of this type from happening again."
The FSA's director of food safety Dr Alison Gleadle said: "We'd like to remind everyone of our usual advice to wash all fruits and vegetables, including salad, before you eat them, unless they are labelled ready-to-eat."
A Morrisons spokesman said: "Morrisons is not the source of this outbreak. We have received no complaints of illness and no Morrisons products have tested positive for Cryptosporidia.
"The HPA's claim is based solely on statistics, not testing. The very same statistics also implicated products from other retailers that the HPA recognise as 'implausible'. Less than half of the cases investigated by the HPA (only 34 people) reported consuming any kind of mixed bag salad from Morrisons. Furthermore, Morrisons shares the same supply chain used for its bagged salads with several other retailers, meaning they would also have been affected by this outbreak."