Some types do, however, make people sick, he said. Those that most commonly cause illness in humans, known as Shiga toxin-producing E. coli, primarily reside in cow intestines, which is why they often contaminate ground beef. The E. coli implicated in the ongoing walnut and ground beef recalls are a type of Shiga toxin-producing E. coli.

Two food recalls this week, ground beef and walnuts, have been linked to E. coli. But what is the bacteria exactly?

Cargill says that, out "of an abundance of caution," it is recalling several of its ground beef products produced in late April and sold at Walmart locations across the eastern U.S.