The CDC warned on Tuesday that Americans should avoid all forms of romaine lettuce because of an E. coli outbreak.

E. coli is a species of bacteria that lives in the intestines of people and animals.

Some E. coli are harmless and live inside our healthy guts, but other strains can make you sick and cause kidney failure or death.

The outbreak the CDC is investigating involves a dangerous E. coli strain that has spread to at least 11 states and Canada. The health agency believes the source of the illnesses is contaminated romaine lettuce.

Once again, if you have any romaine lettuce in the fridge — be it pre-chopped, whole heads, or hearts — the Centers for Disease Control and Prevention says you should throw it out. Now.

"CDC is advising that U.S. consumers not eat any romaine lettuce, and retailers and restaurants not serve or sell any, until we learn more about the outbreak," the agency wrote on its website, just 48 hours out from the biggest nationwide feast of the year — Thanksgiving.

The CDC is investigating an E. coli outbreak that has so far sickened 32 people across 11 US states. Canada's health agency has also reported 18 E. coli cases in Ontario and Quebec. The CDC says the affected people all fell ill between October 8 and October 31, but the agency thinks it's possible that the bad lettuce could still be out there.

So far, no one has died in this outbreak. But the E. coli at play comes from the same family as the one that infected romaine lettuce from the Yuma, Arizona, growing region in the spring. That outbreak killed five people and sickened near 200 others. The fingerprint of this new E. coli, however, is closer to a strain that contaminated leafy greens in 2017; that outbreak killed one person and sickened 25 others in the US.

The O157:H7 strain of E. coli behind all three of these outbreaks can cause kidney failure in about 5% to 10% of cases (usually among people under the age of 5 and over 60, according to the Merck Manual). Less severe symptoms include bloody diarrhea, stomach cramps, and vomiting. So far, 13 people have gone to the hospital and one person developed kidney failure.

The CDC has not linked this outbreak to any region, retailer, or brand of greens, so it is warning anyone who bought romaine recently to get rid of it.

"This advice includes all types or uses of romaine lettuce, such as whole heads of romaine, hearts of romaine, and bags and boxes of precut lettuce and salad mixes that contain romaine, including baby romaine, spring mix, and Caesar salad," the agency said. "Consumers who have any type of romaine lettuce in their home should not eat it and should throw it away, even if some of it was eaten and no one has gotten sick."

Ten illnesses have been reported in California, seven people got sick in Michigan, and other reported illnesses came from Connecticut, Illinois, Massachusetts, Maryland, New Hampshire, New Jersey, New York, Ohio, and Wisconsin.

Here's how an E. coli bug like this spreads and what can you do to reduce your risk of catching the potentially deadly bacteria.

What is E. coli?

E. coli is a rod-shaped type of bacteria that lives in our guts. But some dangerous strains can make you really sick. Flickr/NIAID

E. coli gets a bad reputation as an illness-causing, bad-guy bacteria, but you probably have a bit of good E. coli inside your body right now.

Escherichia coli is a broad term for a species of diverse bacteria. Certain strains of E. coli colonize our guts almost immediately after birth and stick to the mucus of our intestines, keeping our intestinal tract humming along smoothly.

The strain that has been linked to this outbreak, however, O157:H7, tends to be a naughty version of the bug.

What makes the O157:H7 strain so dangerous for people is that it produces a nasty Shiga toxin, which can make us really sick. Shiga-toxin-producing E. coli, or STEC, as it's sometimes called, spreads to humans when little bits of feces get inside our mouths. That's why a bit of undercooked meat or raw milk can make you ill.

O157:H7 usually originates in the guts of cattle, but it can also be found in goats, sheep, deer, and elk. It can travel from cattle farms to nearby fields where lettuce is grown and contaminate the greens in the soil. It can also get on lettuce when food-handlers don't wash their hands properly after coming into contact with E. coli-harboring feces.

The O157:H7 strain sickened 25 people who ate leafy greens including romaine lettuce in November and December, 2017. One Californian died. In May 2017, O157:H7 turned up in soy-nut butter. Thirty-two people got sick from that outbreak, and nine developed kidney failure. Then in the spring of 2018, 197 people across 35 US states got sick on romaine, and a handful of people died.

But contaminated food isn't the only source of potentially illness-inducing E. coli.

"People have gotten infected by swallowing lake water while swimming, touching the environment in petting zoos and other animal exhibits, and by eating food prepared by people who did not wash their hands well after using the toilet," according to the CDC.

Some signs that you've got a Shiga toxin-producing strain of E. coli inside include:

Bloody diarrhea

Harsh stomach cramps

Vomiting

A mild fever

What you can do to reduce your risk

A chopped-salad recipe from the chef Jamie Oliver, a blend of fennel, romaine, endive, and smoked salmon. AP Photo/Matthew Mead

The number-one way to reduce your risk of an E. coli bug is something you've probably heard before: Wash your hands. And make sure people who prepare your food wash their hands, too.

Because E. coli contamination spreads from ingesting little bits of poop, any person or animal along the food chain from field to table can easily contaminate your produce or meat. So it's important to:

Cook meat properly

Keep raw meat separate from other food, and sanitize cutting boards after cutting up raw meat

Enjoy pasteurized dairy and juices, not raw milk

Wash produce before you eat it

Leafy greens (like spinach and romaine) are especially prone to picking up bacteria from nearby cattle farms. Rinsing away extra dirt can help reduce your risk of illness, but if a bit of lettuce is contaminated with E. coli, washing it won't save you.

Pregnant women, young children, older adults, and those with compromised immune systems need to be extra careful about steering clear of E. coli-contaminated food. But most of us can survive E. coli food poisoning — the miserable symptoms usually last five to seven days. There's not much you can do to treat the illness, and typically the best thing to do is try to stay hydrated until it's over.

It can be tough for public-health officials to track the source of an E. coli outbreak to one specific ingredient, but investigators tend to notice certain common foods when they interview sick people about what and where they ate.