Ontario, Quebec, New Brunswick and 12 U.S. states have all been affected by the latest E. coli outbreak. And for the third time this year, the culprit is presumed to be romaine lettuce.

Authorities at the American Food and Drug Administration (FDA) have said they suspect the contaminated lettuce is coming from growers in California’s Central Coast region but have yet to confirm the specific source.

As a result, the Canadian Food Inspection Agency says it will take steps to keep products from areas identified by the FDA from coming into Canada.

Although there’s been no official recall by the Public Health Agency of Canada (PHAC), grocers have pulled the produce off their shelves as a precaution.

According to the PHAC most people who become ill from an E. coli infection will recover completely on their own. Symptoms include nausea, vomiting, headache, mild fever, stomach cramps and diarrhea. Serious symptoms, or those that persist after five to 10 days, should be addressed with a health care professional.

In rare cases, someone may develop life-threatening symptoms, including stroke, kidney failure and seizures, which could result in death. Those most at risk for complications are pregnant women, those with weakened immune systems, young children and older adults.

So, for now, Caesar salad is off the table — literally.

It’s estimated that 20 to 30 per cent of E. coli poisoning is caused by contaminated vegetables — and most often it’s lettuce and other leafy greens.

But why?

One of the reasons those greens are so good for us — high water content — also makes the crop more vulnerable.

“You can get contamination from animal production facilities, it gets into the sediment, it gets into the water, which gets irrigated onto the crops, which are then harvested within 40 to 80 days,” Keith Warriner, a microbiologist specializing in food safety at the University of Guelph, told the CBC.

As reported, produce can also be exposed to the bacteria from birds flying overhead or other animals walking through fields.

And it’s no coincidence that raw produce poses the most risk. Cooking kills E. coli and other bacteria.

What about washing?

Well, it’s included in the PHAC recommendations to reduce risk, as is thoroughly rinsing even”pre-washed” packaged greens with cold water. The agency warns against soaking leaves in a sink full of water as that could lead to contamination from bacteria in the sink itself.

Fruit and vegetable cleaners can give one an extra layer of comfort, but good hygiene may not be … well … quite good enough.

“The bacteria can be stuck on the surface of the lettuce, it can even get inside the lettuce,” Goodridge said. “So if you wash it, you might remove some of the bacteria, but you’re not removing 100 per cent. And we know in some cases, when we look at historical outbreaks of E. coli, even ingesting one single bacterial cell was enough to cause illness.”

To date, there have been 22 confirmed cases of infection in Canada and 48 in the U.S.