Public Health England is investigating an outbreak of Salmonella which has affected 156 people in England over the past few months.
Tests suggest that the cause of the illness, which is often linked to poultry or eggs, is from a single source.
Cases have been seen in Hampshire, London, Cheshire and the West Midlands.
Salmonella can cause diarrhoea, stomach cramps, vomiting and fever. Most people recover without treatment.
The strain of bacteria causing this outbreak of food poisoning, Salmonella Enteritidis, may also have affected people in France and Austria, the PHE said.
Experts are examining 55 cases in Hampshire, 25 in London and 33 in Cheshire.
They are also looking at 43 cases in the West Midlands, 34 of which were connected to an outbreak at Birmingham Heartlands Hospital.
The hospital confirmed three of the people affected have died, but in two cases it was not related to Salmonella. The third has been referred to the coroner's court, but it is not clear on what grounds.
Finding the link
Dr Paul Cleary, a consultant epidemiologist leading the PHE investigation, said: "We are working with our colleagues across Public Health England, at the Food Standards Agency, in local authorities and with other public health organisations in Europe to investigate the cause of this outbreak.
"We are making good progress and hope to have more conclusive evidence shortly.
"We will continue to monitor the situation and if there is any further public health action necessary then we will ensure that this takes place."
Professor Anthony Hilton, head of biological and biomedical science at Aston University, said if the cases across the country were related, the next step would be to identify common risk factors which link the infected individuals.
"This might be consumption of a contaminated food or ingredient or even a common exposure at an event or activity.
"Compiling food and exposure histories of cases occurring over several months can be complex and time consuming, depending on the quality of the information available, and it may be some time before we know the true extent of the outbreak and the causative link, if any."
Public Health England's advice is to wash hands thoroughly before handling, preparing and eating food, and make sure all food is cooked properly before eating.