Health officials are investigating an outbreak of toxin-producing E coli bacteria linked to at least one death in the UK and illness in more than 250 more people. But what is the organism causing the outbreak and how dangerous is it? What is E coli? Escherichia coli is a group of bacteria that normally live in the guts of humans and animals. Most strains are harmless, but some produce toxins that can cause illness, ranging from mild gastroenteritis and fever to severe bloody diarrhoea, stomach cramps and vomiting. Symptoms tend to appear a few days after eating or coming into contact with the bacteria, but can arise within one to 10 days. The illness usually clears up on its own within two weeks. What is behind the latest outbreak? The rise in cases is driven by a type of Shiga toxin-producing E coli, or Stec. In England there are about 1,500 cases of Stec annually. In the month since the latest outbreak was detected on 25 May, the UK Health Security Agency has recorded 275 cases, with more than 100 hospital admissions. Cases have been recorded in England, Scotland, Wales and Northern Ireland and have been traced to lettuce in shop-bought sandwiches. What does Shiga toxin do? The toxin affects the body on multiple fronts, from inhibiting the manufacture of proteins involved in basic physiology to triggering uncontrolled immune responses that affect the kidneys, intestines and central nervous system. How do people become infected? Stec bacteria live in the intestines of cattle, sheep and other animals. Their meat can become contaminated during the slaughtering process, making uncooked meat a common source of infection. But the bacteria also spread through contaminated water and contaminated produce such as lettuce, alfalfa, sprouts, salami, fruit juice and unpasteurised milk. Other routes involve contact with animals or their faeces, or another infected person. The bacteria are highly infectious so can spread swiftly in families and places where people can struggle to stay clean, such as nurseries, schools, nursing homes and hospitals. Who is most at risk? Anyone can contract a Stec infection and many people will have only mild illness. But children and older people are most susceptible to severe infections. In some people, mainly children under five, the infection can lead to haemolytic uraemic syndrome (HUS), a cause of life-threatening kidney failure. Adults can develop a similar, rare condition called thrombotic thrombocytopaenic purpura (TTP). This causes blood clots in small blood vessels around the body. The clots can slow or stop blood flow in the brain, kidneys and heart. As of 25 June, about half of those infected in the outbreak have been hospitalised. Two people died in May within 28 days of their infections. While both had underlying health conditions, only one is thought to have died as a result of the infection. According to Paul Hunter, professor in medicine at the University of East Anglia, the death rate from Stec infections is about 0.5%, with most occurring in people aged 65 and above. About 10% of infections can lead to HUS, which carries a fatality rate of up to 5%. How should I avoid infection? Fewer than 100 Stec cells may be enough to cause disease, so good personal hygiene, ensuring salad vegetables are properly washed, and that meat is well cooked and not left out are all important precautions. As ever, wash hands after using the toilet, changing a nappy, handling raw meat, before meals and after contact with animals. Do not swim in water that might be contaminated by cattle or sheep in nearby fields. Children and older people, who are most at risk of serious illness, are advised to avoid unpasteurised milk and other dairy products. Is the outbreak over? While the peak of the outbreak seems to have passed, more cases are likely and there may be further deaths. Leafy salads are a well-known risk for Stec infections and are difficult to avoid, says Hunter, who believes it may be impossible to work out exactly how the lettuce became contaminated or even where it was grown. “New infections, from this source at least, seem to have been controlled, but of course Stec infections are not that rare and will probably continue to be reported,” he says.