As Walmart, Trader Joe's, and Whole Foods recall salads, here's what a food-poisoning expert says you should avoid buying

Marler told Business Insider that the idea that he would have to warn people against drinking unfiltered, untreated water didn't cross his mind until this year.

"Almost everything conceivable that can make you sick can be found in water," Marler said.

So-called "raw water" — even from the cleanest streams — can contain animal feces, spreading Giardia, an intestinal infection that includes symptoms such as vomiting and diarrhea and results in roughly 4,600 hospitalizations a year.

E. coli, cholera, and hepatitis A, which led to 20 deaths last year in an outbreak in California, can also be spread through untreated water.

Sprout-related outbreaks are surprisingly common, with more than 30 bacterial outbreaks— primarily salmonella and E. coli — in the past two decades.

"There have been too many outbreaks to not pay attention to the risk of sprout contamination," Marler says. "Those are products that I just don't eat at all."

Uncooked flour is at the other end of the spectrum — something most people see as harmless but that can actually spread bacteria, Marler says.

Citing a study in the New England Journal of Medicine, Consumer Reports said that from late 2015 to September 2016, 56 people in 24 states developed an E. coli infection from eating raw or uncooked flour.

Marler says that while most people think raw eggs are the biggest food-poisoning threat in cookie dough, the flour can also be a culprit. And you don't even have to eat it — simply not washing your hands after getting uncooked flour on them can help spread E. coli bacteria.

Marler says he has seen more foodborne illnesses linked to shellfish in the past five years than in the two preceding decades.

The culprit? Warming waters, he says.

As global waters heat up, they produce microbial growth that can end up in the raw oysters that consumers slurp down.

Marler and President Donald Trump have at least one thing in common: They are ordering their steaks well-done.

According to Marler, meat needs to be cooked to 160 degrees throughout to kill bacteria.

For anyone who remembers the salmonella epidemic of the 1980s and early '90s, this is a no-brainer.

According to Marler, the chance of getting food poisoning from raw eggs is much lower today than it was 20 years ago. But he still isn't taking any chances. However, in April, the FDA announced a recall of 206 million eggs over salmonella-contamination concerns.

A precursor to the raw-water trend was the movement encouraging people to drink "raw" milk and juices, arguing that pasteurization depletes nutritional value.

Marler says pasteurization is not dangerous, but raw beverages can be, as skipping the safety step means an increased risk of contamination by bacteria, viruses, and parasites.

"There's no benefit big enough to take away the risk of drinking products that can be made safe by pasteurization," he said.