The Health Protection Agency (HPA) said an investigation into an outbreak of Cryptosporidium infection in England and Scotland showed strong evidence of a link with eating pre-cut bagged salad.
About 300 people are known to have been affected by the outbreak of the infection last May which principally causes diarrhoea. The upsurge of the illness was first spotted in Yorkshire, which reported 10 times the usual number of cases.
The HPA said the strongest association was found with eating pre-cut mixed salad leaves bought at supermarket chain Morrisons and to a lesser extent from Asda.
Despite detailed investigations, the source of contamination was not pinpointed.
Stephen Morton, Yorkshire director of the HPA, 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.”
A Food Standards Agency spokeswoman 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’.”
Morrisons said it was not the source of the outbreak. It had received no complaints of illness and no products had tested positive for the bacteria.