The Centers for Disease Control and Prevention (CDC) is investigating McDonald’s Quarter Pounder hamburgers amid reports that an E. coli outbreak linked to the sandwiches is making Americans sick.

The investigation comes amid a rash of foodborne illness cases in which deli meat, waffles, eggs, and other foods tainted with E. coli, listeria, and salmonella have been responsible for product recalls, hundreds of hospitalizations, and even deaths across the United States.

There are two primary reasons for the recent uptick in announcements of tainted food. One, the US food system has become extremely complicated in recent decades: There are more imported foods now as well as more highly processed foods, which creates more opportunities for disease to enter the food system.

Two, the government has better and faster tracing capabilities, thanks to legislation around food safety modernization. That makes it easier for the Food and Drug Administration and the US Department of Agriculture, the two bodies that investigate such outbreaks, to track problems to their source. It also makes it easier for companies to recall tainted products before they spread further into the food system and sicken large numbers of people.

Here is what you need to know about the most recent cases of foodborne illness and why these outbreaks are happening.

The latest issue with tainted food is at McDonald’s, where E. coli linked to Quarter Pounders has spread to 10 states and sickened 49 people — although according to the CDC, the actual number of people who got sick from this outbreak is probably much higher. The exact source of the outbreak is under investigation, but the CDC believes onions or beef used in the burger could be the cause.

E. coli, a type of bacteria, isn’t dangerous in and of itself. As Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security, told Vox, “You have E. coli in your body. I have E. coli in my body,” specifically in the gastrointestinal tract. The problem is, some strains make people sick and are especially dangerous when they get into a water supply. Tainted water used in irrigation for agriculture can contaminate food. Deadly strains can also be introduced into meat products, particularly ground beef, when the animal is slaughtered and the meat is processed. E. coli can also spread by person-to-person contact if a sick person is not careful about hygiene.

Unfortunately, E. coli isn’t the only bacteria affecting the food supply right now. Listeria, which spreads easily and rapidly and can only be killed by heating food to high temperatures, led to a Friday recall of a wide range of waffles produced for various national supermarkets. This comes after a slew of chicken products at grocery stores like Trader Joe’s and Aldi were also flagged for recall due to listeria concerns tied to BrucePac, a pre-cooked meat producer.

Both of these recalls follow the listeria outbreak that occurred in Boar’s Head meat products, including liverwurst, that were produced in June and July.

The outbreak originated at the company’s Jarratt, Virginia, facility and has been found in 19 states. It has killed 10 people out of 59 known cases so far, according to the CDC. Concerns about unsanitary conditions at the facility, including mold, vermin, and meat and fat residue on equipment, floors, and walls go back at least two years.

Boar’s Head was forced to recall 71 products and 7.1 million pounds of deli meat as a result of the outbreak. The company closed the Jarratt facility indefinitely and permanently stopped production of their liverwurst.

The third contaminant of concern at the moment, salmonella, was the subject of a September egg recall as well as a more recent recall of green onions. It comes primarily from poultry, like chicken and eggs, but it can show up in other types of food. In 2024, salmonella outbreaks were linked to eggs, basil, charcuterie meat, and cucumbers. The outbreaks linked to eggs and charcuterie meats had the highest numbers of hospitalizations, and each spread to more than 30 states.

“If you go to your grocery store right now and buy some chicken and you culture it, you’ll find salmonella,” Adalja said. “It has an easy route, if someone doesn’t cook their food appropriately. It can even be happening in your own home; you don’t cook your chicken all the way through or you’re cutting chicken on a cutting board and then you start cutting lettuce or something. You can cross-contaminate.”

Listeria, salmonella, and E. coli infections are particularly concerning for the disproportionate effect they can have on vulnerable groups like pregnant people, children, and people over the age of 65. Healthy individuals not in these groups may experience symptoms after exposure to such bacteria, but they’re less likely to need treatment for them. And in many cases, people who experience symptoms may well be able to recover on their own.

As the Boar’s Head outbreak made clear, however, these bacteria can lead to more severe cases and even fatalities. Listeria causes about 1,500 hospitalizations per year, salmonella about 26,500, and E. coli about 3,300.

Listeria is of particular concern as it is “just a much more virulent pathogen when it comes to infection than, for example, salmonella,” Adalja said. “It disseminates very quickly, and sometimes it doesn’t cause telltale signs people think of foodborne illness causing: vomiting and diarrhea. Listeria might not cause that. It may just present as a systemic infection or with meningitis.”

Below is a rundown of the symptoms that each of these bacteria can cause and who’s most at risk:

The latest alerts add to a steady uptick in food recalls since the pandemic. In 2023, the total number of food recalls and public health alerts by the USDA and FDA were the highest they’ve been in the last few years, according to a report from the US Public Interest Research Group, a consumer advocacy group. (They’re still lower than they were from 2018 to 2020, however.)

These recalls were largely driven by companies’ poor disclosure of allergens in different foods, with a smaller fraction due to the discovery of bacteria in items. (The USDA and FDA have said the number of food products they’ve recalled in 2024 have not increased compared to 2023.)

Recent recalls are driven by a couple of factors, including lengthier and more complicated food supply chains. Basically, if there are more steps involved in processing and packaging food, there are more chances for germs to spread and items to get contaminated. The popularity and prevalence of processed food, which is a major source of such pathogens, is also an issue. And better testing and detection means that authorities are able to more frequently identify these problems.

“Our supply system has become so complicated that there are more and more blind spots that are allowed to exist and allowed to be ignored throughout the system,” Darin Detwiler, a food policy expert at Northeastern University, told Vox.

There’s also a growing use among government agencies of what’s known as culture-independent diagnostic tests, which are more sensitive to pathogens and can turn results around more quickly than previous methods. The expanded use of these tests suggests that outbreaks could be getting caught with greater regularity and contributing to the sense that more are taking place.

Information technology is also playing a role, Detwiler notes. The FDA is pushing for more digitization in tracing outbreaks of foodborne illnesses, which enables a faster response to the problem.

There can be repercussions for those responsible for outbreaks, though that’s often in the form of a fine, which can be negligible for large corporations. Boar’s Head is facing a slew of lawsuits related to the listeria outbreak, and the US Department of Agriculture, which is responsible for inspections of food production facilities, is now facing an internal investigation into its role.

Even though the burden of making sure food is safe should fall on the companies that produce and handle it, as well as the government bodies tasked with inspection, Detwiler cautions that consumers should be more aware of the risks their food poses.

“We need to assume that food is always contaminated,” he said. “We always have to be vigilant about … where we buy, what we buy, what we eat, how we prepare the food, the questions we ask, that kind of thing, to be a stakeholder in that process as well.”