The most basic of baking ingredients, flour, is at the center of a complicated question: How did it become contaminated with bacteria normally found in animal feces?

The E. coli-tainted flour from General Mills has sickened 46 people in 21 states and prompted about 45 million pounds of it to be recalled. And, the Food and Drug Administration is cautioning raw cookie dough and cake batter aficionados not to indulge.

Here's what federal officials know:

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HOW FOUL IS THE FLOUR?

FDA testing determined that raw dough eaten or handled by people who became ill was made using flour was produced between November 2015 and February 2016 at the General Mills facility in Kansas City, Missouri.

Officials found two different strains of E. coli bacteria that produce a toxin that can make people sick. In the past several years, the CDC has investigated several multi-state outbreaks of these E. coli strains linked to sources such as sprouts, but never raw flour.

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HOW DID THE E. COLI GET INTO THE FLOUR SUPPLY?

The CDC and FDA don't know, though they are investigating.

The bacteria lives in animal intestines and is most commonly spread through contamination from feces. Flour is refined from wheat grown in open fields and it is considered a raw agricultural product with no treatment process designed to kill contaminants like bacteria before it reaches consumers. As a result, there's potential for contamination between the field and the packaging, CDC epidemiologist Dr. Karen Neil said.

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WHAT TYPES OF ILLNESSES ARE REPORTED AND WHERE?

These strains of E. coli bacteria can cause diarrhea and abdominal cramps. Most people get better within a week, but some become seriously ill and develop potentially fatal kidney failure. Thirteen people have been hospitalized and one has kidney failure, the CDC said.

Minnesota, where General Mills is based, has had the most illnesses with five. Colorado, Illinois, Michigan each reported four.

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WHICH PRODUCTS ARE AFFECTED?

The recall is a small fraction of the 2.5 billion pounds of flour General Mills produces in a year.

Batches of unbleached, all-purpose, and self-rising flour varieties from General Mills, Gold Medal, Wondra and Signature Kitchens flour brands are included. A few cake mixes, a biscuit mix and a pancake mix containing General Mills flour also were recalled. All recalled products can be found here: http://bit.ly/2arKNqx .

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WHAT DOES GENERAL MILLS SAY?

In an emailed response to The Associated Press, the company said its officials do not believe the Kansas City plant is the source of the bacteria but they "have elevated cleaning protocols" out of caution.

"Only a small sub-set of flour produced at the Kansas City plant has been traced back to individuals who have become ill. To date, E. coli has not been found in testing of the manufacturing facility," the company said.

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IS THIS THE FIRST TIME FLOUR HAS BEEN THE SOURCE OF FOOD-BORNE ILLNESS?

Raw flour, yes.

In 2009, an E. coli outbreak that sickened 72 people in 30 states was traced to prepackaged Nestle Toll House cookie dough that consumers reported eating raw. Neil said the likely culprit was the flour in the dough.

Nestle now uses heat-treated flour, and commercial cookie dough used in ice cream and other ready-to-eat products also is treated. General Mills said it can't heat-treat flour because it would affect the rising properties and other baking performance.

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BUT I LOVE COOKIE DOUGH, WHAT NOW?

Throw away any product included in the recall, including flour stored in containers other than the original packaging, Neil said.

Stop making homemade playdough for children or tasting cake or cookie batter before cooking.

Cooks at home and in restaurants and bakeries should wash hands, work surfaces and utensils thoroughly after working with raw dough products containing flour. Keep raw foods away from flour to prevent cross-contamination.