The bacteria Listeria monocytogenes can turn up in all types of food: salad dressings, soft cheeses, cold-cut meats, fresh vegetables and more.

This month, the Centers for Disease Control and Prevention said that at least 33 people have been sickened in a listeria outbreak linked to meat sliced at deli counters. And in June, dozens of ice cream products were recalled because they may have been contaminated with the bacteria.

Most people who ingest listeria don’t get very sick, if they develop symptoms at all. But certain high-risk individuals can fall seriously ill. Approximately 1,600 people in the United States develop serious listeria infections, known as listeriosis, every year — and about 260 of those infections are fatal, said Laura Gieraltowski, an epidemiologist with the C.D.C.’s outbreak response and prevention branch.

Here’s what you need to know about listeria to stay safe.

How does listeria end up in food?

Listeria bacteria are naturally found in the soil, which means they also end up on or inside things that come into contact with the soil, such as plants and animals.

“We find it pretty much everywhere in nature when we look for it,” said Catherine Donnelly, an emeritus professor at the University of Vermont who has studied listeria for decades.

The bacterium is also “a hardy germ, and it is tough — it can survive for a long time,” Dr. Gieraltowski explained. Food contaminated with listeria can deposit the bacteria on counters, deli slicers and other surfaces where food is prepared and processed, where it can linger and contaminate other foods.