As public health officials scramble to pinpoint the source of a deadly outbreak of a new strain of E. coli in Germany, a growing list of illnesses and advisories in Europe has sparked fears of food around the world.
For consumers, the outbreak also raises questions about which foods are riskiest to eat and why.
But ranking risk is difficult to do, experts say, partly because of the way statistics are tracked and partly because most cases of food-borne illness go unreported or uninvestigated. And any type of food can be a vector for dozens of varieties of infectious bacteria. Still, there are some trends.
Among the worst offenders are undercooked meat, chicken and eggs. Fresh produce is also a major cause and a focal suspect for ongoing investigations in Germany. Public health agencies have warned people against eating raw sprouts, tomatoes, cucumbers and leafy greens that were grown in northern Germany.
In cases of contaminated produce, the fecal matter of animals is always ultimately to blame. Sometimes, farmers apply bacteria-laden manure directly to agricultural fields.
In other cases, irrigation water becomes contaminated. Wildlife may enter fields. Or human workers can contaminate food supplies during handling activities.
The first key to staying healthy, experts say, is to recognize what we're up against.
"We live in a world surrounded by bacteria," said Timothy Jones, state epidemiologist at the Tennessee Department of Health in Nashville. "Sometimes, epidemiologists joke about the fecal veneer that's everywhere. It's sort of a gross thought, but really, that's the world we live in."
Every year, one out of six Americans gets sickened from food that is contaminated with bacteria, according to the Centers for Disease Control and Prevention. The CDC documents more than 1,000 outbreaks each year for a total of 48 million people who develop symptoms ranging from nausea and abdominal cramping to fevers and severe diarrhea. Three-thousand people die annually from food-borne illnesses.
There are 31 known kinds of bacteria, viruses and other pathogens that infect people through food. But only a handful of agents cause the majority of diagnosed cases. (Many millions of cases have unknown causes.)Salmonella causes the most cases that are ultimately solved. E. coli 0157 causes the most severe outbreaks. Other players include norovirus, Listeria and Campylobacter.
Each type of infectious agent has its own preferred habitats. Campylobacter, for example, likes to live in chickens and other poultry, while E. Coli 0157 is happiest inside a cow.
That explains why ground beef has long been the top cause of E. coli 0157 infections, though improved regulations have reduced the percentage of cases caused by beef. Instead, produce has taken on a growing share of responsibility for spreading the bacteria.
Any type of fruit or vegetable can become contaminated with E. coli, Salmonella or other infectious microbes. But certain varieties have been most often linked to outbreaks, including sprouts, leafy greens, tomatoes, jalapeño peppers and melons.
Sprouts are particularly concerning because outbreaks usually start with contamination of the seeds, which are placed into warm water to make them sprout.
"You have an agricultural product with some risk of having E. coli or Salmonella in the seeds," said Kirk Smith, supervisor of the Food-borne Diseases Unit at the Minnesota Department of Health in St. Paul, who recommends that people avoid eating sprouts altogether. "And then you're incubating them in the same conditions that make bacteria grow."
Once they're big enough to eat, sprouts have countless nooks and crannies that are impossible to wash well. Even if you could power-wash the crunchy salad-toppers, bacteria may become embedded inside the plant tissue. The same thing can happen with spinach, lettuce, tomatoes and other produce. In these cases, no amount of washing can remove the contamination.
Produce is also problematic because we eat it raw. It is easy to kill bacteria in meat and eggs as long as you cook them well and at high enough temperatures. But nobody boils a head of romaine before making sandwiches.
To reduce your risks of getting sick from your food, the CDC recommends washing hands in warm, soapy water for at least 20 seconds before cooking. Avoid cross-contaminating knives and cutting boards. And wash all produce thoroughly, even if it claims to be pre-washed.
The only foods Smith won't eat are sprouts, undercooked meat, raw milk, raw oysters and unpasteurized juices, like apple cider. Otherwise, he said, it's worth taking food-borne illnesses seriously, but it's better to eat fruits and vegetables than to avoid them out of fear.
"Things aren't perfect and they're never going to be perfect, but they're better than they've ever been in history," Jones said, adding that 3,000 deaths a year compared to billions of meals served every week adds up to a miniscule statistic.
"What kills most people in this country is obesity, heart disease and cancer," he said. "Vegetables and a healthy diet are very important for all of those things."