Listeria most commonly infects chilled, ready-to-eat foods such as pre-packed sandwiches, pate and soft cheeses.
In rare cases it leads to listeriosis, which can cause a temperature, vomiting and aches and pains.
Most people will get better on their own but for some the illness will be fatal. Hospital patients are especially vulnerable to listeria’s effects as they are usually elderly or have underlying health conditions.
All patients who died or became ill are believed to have eaten chicken salad sandwiches, which have since been withdrawn while investigations continue.
Listeria monocytogenes (stock) as found in the pre-packaged food sold at hospitals
The source has been traced to North Country Cooked Meats, which is based in Salford, and The Good Food Chain is now considering suing them. Yesterday, director Martyn Corfield, 50, said: ‘My sincere thoughts are with the families affected. I feel for them because I have been through it, I’ve been bereaved. But I’m very angry too because we are a victim as much as anyone else.’
Dr Nick Phin, of Public Health England said that ‘any risk to the public is low’.
Most people that catch listeriosis, caused by bacteria called listeria, will only experience mild symptoms, such as vomiting and diarrhoea.
Other symptoms of the infection can include a high temperature of 38C or above, aches and pains, and chills, according to the NHS.
However, more serious complications can develop in those with weakened immune systems, babies, the elderly and pregnant women.
Many foods can harbour listeria, but it is usually found in unpasteurised milk, soft cheeses and ready-to-eat foods, such as prepacked sandwiches.
Listeria is widespread in the environment and can be found in raw food and soil, and in the droppings of many mammals, birds, and fish.
Around 180 cases of listeriosis are confirmed every year in England, according to figures. It strikes around 850 annually in the US.