An outbreak of E. coli linked to contaminated salad vegetables has caused at least 22 deaths and hundreds of infections in Germany, Sweden and other countries.
What is E. coli?
E. coli is short for Escherichia coli. It is a type of bacterium present in the gut of humans and other animals.
Most strains are harmless but some are able to produce toxins that can cause symptoms in humans.
The severity of the illness people can get varies considerably, but some types of E. coli lead to severe cramps and diarrhoea.
Previous outbreaks have been linked to the O157 strain, however, this seems to be something different.
What do we know about this strain?
Initial tests looking for "markers" on the surface of the bacterium showed it to be the O104 strain, which the World Health Organization said was rare, had been seen in humans before, but never in an outbreak.
When researchers looked at all the genes inside the bacterium they concluded that it was in fact a new strain of O104.
Bacteria are able to exchange genes between different strains and species.
Experts believe this variant has acquired a deadly combination of producing a toxin which can damage the kidneys and being really good at sticking to the gut, allowing more bacteria to grow and ultimately produce even more toxin.
What are the health effects?
Symptoms - such as diarrhoea with blood in it, severe cramps, and fevers - can take up to eight days to develop.
It is hoped that the number of cases will begin to fall following public health advice issued in Germany last week.
But the outbreak is causing severe infections and in a number of cases, affecting the blood and kidneys.
Haemolytic uraemic syndrome - an unusual complication of some types of E. coli as well as other infections - has been seen in hundreds of current cases.
This can be mild, however, in some cases it can lead to epileptic fits, permanent kidney failure and even death.
Large numbers of people between the age of 16 and 60 have developed haemolytic uraemic syndrome (HUS).
"It is extremely unusual for people in these ages groups to develop HUS," the Health Protection Agency says.
What is the health advice?
The most likely source of the outbreak is a bean sprout farm in northern Germany.
Officials in the country are awaiting full results of further tests to confirm this, but are advising people to stop eating bean sprouts.
People are still being advised to avoid eating raw tomatoes, cucumbers and leafy salad including lettuce, especially in the north of the country, until further notice.
The vast majority of cases come from contaminated food produce, although it is possible to spread from person to person. Good hand hygiene can prevent this.
The UK's Health Protection Agency says anyone returning from Germany with illness including bloody diarrhoea should seek urgent medical attention and mention their travel history.
Treatment will probably include fluids and pain killers. There is evidence that some common antibiotics might not work and some experts believe antibiotics could make the illness worse. For patients with severe HUS, dialysis might be needed to support failing kidneys.
The Food Standards Agency in the UK has issued general advice on the need to wash fruit and vegetables.
The agency says: "It's a good idea to wash fruit and vegetables before you eat them to ensure that they are clean, and to help remove germs that might be on the outside.
"Peeling or cooking fruit and vegetables can also remove these germs."
However, a Scottish expert said new research suggests washing alone may not be enough, as the bacteria may be inside the food.
Dr Nicola Holden of The James Hutton Institute said: "The bacteria are able to get from animal sources on to crops through different routes, most likely in irrigation water or sometimes from slurry spraying, while some contamination can also occur during processing and packaging."
She said the bacteria can colonise plant roots, moving up to the edible foliage or fruits.
"The threat to human health occurs because these bacteria are not simply sitting on the surface of the plant and are particularly difficult to remove post-harvest," she added.
What do other experts say?
Professor Brendan Wren from the London School of Hygiene & Tropical Medicine said E. coli can attach to the surface of fresh produce.
"These types of E.coli survive harsher environmental conditions than the typical E. coli and produce some nasty toxins to humans," he said.
"They can survive in soil environments and fertiliser may be one source for the origin of the outbreak."
Dr Jonathan Fletcher, senior lecturer in microbiology at the University of Bradford, said toxin-producing E. coli can cause very serious disease in humans, especially in the elderly or very young.
Cattle seem to carry the toxin in their gut, without showing signs of illness, and it will be shed in the faeces.
"If cattle manure is used as a fertiliser, it is probable that vegetables will be contaminated with E. coli, and if not washed properly it would be present in sufficient numbers to cause the infection."
What about the UK?
The total number of cases in the UK is 13.
Three have developed the potentially deadly complication of haemolytic uraemic syndrome.
It seems all picked up the infection in Germany. The HPA said there was no evidence of E. coli being passed from person to person in the UK.