LONDON (Reuters) – The number of people to die from a listeria outbreak in British hospitals has risen to five from three, Public Health England (PHE) said on Friday.

The agency said that since the outbreak it had reviewed earlier cases and established that one other death had also been linked to the infection, which is thought to stem from sandwiches and salads. One patient who had been seriously ill has also since died, taking the number to five.

Sandwiches and salads from the Good Food Chain linked to the outbreak have been withdrawn and production stopped.

All but one of the deaths happened more than a month ago, PHE said.

The chain – which supplied 43 NHS trusts across the UK – had been supplied with meat produced by North Country Cooked Meats, which subsequently produced a positive test result for the outbreak strain of listeria.

“To date, there have been no patients linked to this incident outside healthcare organizations, but we continue to investigate,” said PHE’s Nick Phin. “Swift action was taken to protect patients and any risk to the public is low.

What is listeria?

Listeria is a bacterium that can cause a type of food poisoning called listeriosis.

Normally, the symptoms are mild – a high temperature, chills, feeling sick – and go away on their own after a few days.

But in this outbreak, the cases occurred in people who were already seriously ill in hospital and they are most at risk of severe infection.

Listeria can then cause damage to organs, spread to the brain or bloodstream and be fatal.

In 2017, figures show there were 33 deaths linked to listeriosis in England and Wales.

Many types of food can become contaminated with listeria such as soft cheeses, chilled ready-to-eat foods like pre-packed salads, sandwiches and sliced meats, and unpasteurised milk products.

Pregnant women are advised to steer clear of soft cheese for this reason.

To reduce the risk, the NHS advises people keep chilled food in the fridge, heat food until it is piping hot and not eat food after its use-by date.

The Good Food Chain, based in Stone, Staffordshire, had been supplied with meat produced by North Country Cooked Meats, which subsequently produced a positive test result for the outbreak strain of listeria.

This business – along with North Country Quality Foods which it distributes through – has also voluntarily ceased production.

Last week North Country Cooked Meats said it was “co-operating fully” the investigations.