The promise of the American food supply is that you can eat anything and not get sick. You can usually assume that whatever you buy from a grocery store or fast-food joint won’t land you in a hospital.

But lately, foodborne-illness outbreaks seem to be distressingly regular. On Tuesday, the CDC reported 49 cases and one death linked to McDonald’s Quarter Pounders tainted with E. coli. In the past week, hundreds of waffle and pancake products were voluntarily recalled due to potential Listeria contamination. Listeria in particular has been a problem of late: Earlier in October, more than 11 million pounds of ready-to-eat meat and poultry products were recalled. And an especially bad Listeria outbreak involving Boar’s Head deli-meat products has led to 59 hospitalizations across 10 states and 10 deaths.

Many of this year’s outbreaks have occurred in foods that are preprepared—those that can be eaten as-is, without further cooking. Foods such as Quarter Pounders and waffles, yes, but also cold cuts, prepackaged salads, and jarred salsa are popular because they are convenient. That convenience comes at a cost. A rule of thumb in food safety is that “the more a food is handled prior to consumption, the higher the chances it can be contaminated,” Lawrence Goodridge, the director of the Canadian Research Institute for Food Safety, told me. Americans are left with a difficult choice: save time or risk getting sick.

Many bacteria that cause foodborne illness live among us. Listeria can be found in soil and water, and E. coli and Salmonella are normally found in human and animal digestive tracts. They become a problem when they get into food. Preprepared foods are particularly prone to contamination because they are usually processed in large, sometimes even multiple, facilities where microbes have lots of opportunities to spread. “Somebody, somewhere, or a company, has produced the food so that we don’t have to do it at home,” Goodridge said. A factory worker with mud on his shoe, or an employee who didn’t wash her hands after using the bathroom, can be all it takes to start an outbreak. Food-safety practices—such as regular cleaning, temperature control, and strict hygiene standards—are supposed to keep these factories pristine. But occasionally, they fail.

Refrigerated facilities keep most bacteria at bay—microbes grow more slowly at lower temperatures—but not Listeria, which thrives in cool conditions. Given enough time to grow, a Listeria colony forms a protective gel over itself, called a biofilm, which makes it especially difficult to get rid of. Meanwhile, E. coli typically gets into produce through water soiled with feces. Usually, contamination occurs at the farm level, but microbes can spread as fresh foods are processed into products such as precut fruit, bags of chopped lettuce, and even prewashed whole greens. When clean produce is washed together with a contaminated batch or sliced with the same equipment, bacteria can spread. Many foods are produced in a central location and then shipped cross-country, which is how a contamination event at a single farm can lead to illnesses nationwide.

This may be the reason for the ongoing Quarter Pounder debacle. According to McDonald’s, the E. coli outbreak may be linked to slivered onions, which were sourced from a single supplier that served certain McDonald’s locations in 10 states, as well as some Taco Bell, KFC, and Pizza Hut stores. Centralizing the slivering of onions no doubt increases efficiency at fast-food chains. But it also raises the risk of contamination.

In food safety, cooking is known as a “kill step,” because high heat kills most dangerous pathogens. Precut salads and fruit are usually eaten raw. Nobody cooks cold cuts, even though the CDC recommends heating them until they are steaming (who knew?). Even convenience products that are meant to be heated, such as frozen waffles and vegetables, aren’t always prepared properly at home. A toaster may not get a waffle hot enough—Listeria is killed at an internal temperature of 165 degrees Fahrenheit—and thawed frozen vegetables may be eaten without being boiled first, Barbara Kowalcyk, a food-safety expert at George Washington University, told me.

To be clear, there’s no need for Listeria hysteria. “On the surface, it looks like there are many more outbreaks,” but there are no data to prove that yet, Goodridge said. Still, some recent outbreaks demonstrate that precautions are working as they should. Listeria was identified in a regular sweep of the waffle factory and products were voluntarily recalled; no cases of illness have been reported. Tools for detecting outbreaks are becoming more sophisticated, Darin Detwiler, a food-safety expert at Northeastern University, told me. A technique called whole-genome sequencing can identify instances in which people have been sickened by the same bacteria, pinpointing the source of an outbreak. Earlier this year, it was used to investigate a Listeria outbreak in Canada that killed three people and hospitalized 15.

No food is totally safe from contamination. Practically everything sold in stores or restaurants is handled in some way. Milk is pooled from any number of cows, then pasteurized and packaged. Hamburger patties are usually made with meat from many butchered cows that is then ground, seasoned, and formed. People get lulled into the idea that “the U.S. has the safest food supply in the world,” Kowalcyk said, “but that doesn’t mean that it’s safe.” People can reduce their risk of contracting a foodborne illness by buying whole foods and cooking from scratch when possible, Goodbridge said; it’s probably safer to clean and chop your own head of lettuce. Yet even that is not a guarantee. Foodborne illness also spreads in home kitchens, where cross-contamination of raw meat with other foods, unsafe storage, and food spoilage often occurs. The risks are lower for healthy people, who can usually get through foodborne illness without excessive discomfort. But for vulnerable groups—very young, very old, and pregnant people—foodborne illness can lead to hospitalization, and even death.

The recent spate of outbreaks highlights the dilemma plaguing the state of American eating. People are simply too busy and exhausted to cook from scratch. In the daily scramble to get dinner on the table, ready-to-eat food is a lifeline. But with every additional stage of preparation comes an extra helping of risk.