This outbreak is still evolving, and the CDC, FDA, and U.S. Department of Agriculture are all investigating where the exact source might be, as it's likely to be a particular beef supplier. E. coli can hang out in a number of foods, from ground beef to flour, but leafy greens are actually one of the more common sources. That's not necessarily to say that lettuce and sprouts are more likely to harbor the bacteria. E. coli comes from animal intestinal tracts, so improper hygiene on farms can enable the bacteria to spread either through direct contact with fecal material or, more often, through contaminated water running off livestock pastures. Unpasteurized, or so-called "raw," dairy products are also a fairly common source—the whole point of inventing pasteurization was to kill the bacteria hanging out in milk. E. coli can get pretty much anywhere, but it's leafy greens that make the most people sick. Beef, pork, and chicken all have to be cooked before you can eat them, but you're probably not cooking your salad. That's why the CDC was so quick to warn people against all romaine during the Thanksgiving outbreak in 2018: if people wound up with contaminated lettuce, they were almost certainly going to eat contaminated lettuce. But with a little precaution, even those who unwittingly buy contaminated beef should be okay.