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The parasite has sickened thousands across the United States. Experts have tips on how to stay safe.

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The United States is in the midst of its worst year yet for cyclosporiasis, an intestinal illness caused by a parasite transmitted through food or water contaminated with human feces.

Federal officials have linked at least 1,644 cases of the illness in five states — Indiana, Kentucky, Michigan, Ohio and West Virginia — to shredded iceberg lettuce served at Taco Bell. In a notice posted Thursday night, the Centers for Disease Control and Prevention warned people not to eat lettuce from Taco Bell locations in those states.

Of the cases linked to Taco Bell, 94 people have been hospitalized, according to the C.D.C.

According to two federal officials who declined to be named, the C.D.C. has linked the outbreak to Taylor Farms, which supplied the iceberg lettuce to Taco Bell. The firm, based in Salinas, Calif., is one of the largest producers of fresh vegetables and fruits in the country, and their products — including bagged lettuce and salad kits — are sold in many U.S. supermarkets including Safeway, Whole Foods, Target and Walmart.

Federal officials have not publicly named Taylor Farms in the outbreak, but the Food and Drug Administration said Thursday night that it had traced the lettuce to a single supplier from Mexico. As the investigation continues, other brands, restaurants and stores may be identified in the outbreak, the agency said.

In a statement Thursday night, Taco Bell said it was voluntarily removing potentially affected lettuce from its locations in select states. It did not identify Taylor Farms as the supplier of the lettuce, but said lettuce from the implicated supplier would be “indefinitely removed” and replaced quickly from its supply chain nationwide.

This is now the largest recorded cyclospora outbreak in U.S. history, and case counts are on track to exceed that of any previous year. Since 2016, the C.D.C. has reported an average of about 2,800 cases per year nationwide.

On Tuesday, the C.D.C. reported 1,645 confirmed cases across 34 states, which was nearly double the agency’s case count from last Friday. It also reported 141 hospitalizations. Officials said they were aware of more than 5,100 additional cases nationwide that they had not yet confirmed.

The agency’s figures are most likely an undercount, health experts say, as some people who are ill are never diagnosed. When they are, it can take time for states to report cases to the agency and for the agency to confirm them.

Cases are expected to rise, possibly through August, said Gwen Biggerstaff, a health official at the C.D.C., in a news conference on Tuesday.

Michigan is currently reporting the most infections in the country: 5,002 cases and 102 hospitalizations as of Friday, the highest number of cases ever reported in Michigan in a single year. The state typically records only 40 to 50 cases per year.

Other states with higher than usual case counts include North Carolina, Illinois, Indiana, Maryland and Texas.

What is cyclospora?

Cyclospora is a microscopic parasite that infects people when they consume food or water contaminated with its oocysts (or eggs), said Keith R. Schneider, a professor of food safety at the University of Florida.

It’s normal to see more cases this time of year, he said. They tend to rise from May through August in the United States, because the parasite thrives in warm temperatures. But the spike in cases in Michigan and many other states is unusual and suggests we are in the midst of one or more large outbreaks, Dr. Schneider said.

After ingesting the parasite, it can take from two days to two weeks or more for it to infect the intestinal lining and cause symptoms, including cramping and watery diarrhea.

An infected person will then shed the oocysts in their feces, but it takes a week or two for the parasite to become infectious outside the body. That lag most likely explains why the illness does not seem to spread directly from person to person, Dr. Schneider said.

How unusual is this outbreak?

Previous cyclospora outbreaks in the United States have been linked to lettuce and salad mixes, raspberries, basil, cilantro, fruit mixes, snow peas and snap peas. Cases often rise during warmer months, and the C.D.C. considers May through August the annual cyclosporiasis season. Even so, the scale of the current outbreak is unusual.

The thousands of cases imply a “massive degree of human fecal contamination,” said Matthew Moore, a food safety microbiologist at the University of Massachusetts Amherst. It is unclear how the human feces contaminated the produce.

The largest outbreak in recent decades occurred in 2020 and sickened 701 people across 14 states, according to the C.D.C. That outbreak was linked to Fresh Express bagged salads. A 2022 Florida outbreak was also linked to bagged salads.

Some who have become ill in Michigan reported eating at restaurants, including fast food chains, but not everyone reported eating out, said Natasha Bagdasarian, the chief medical executive for Michigan. Produce is probably contaminated in fields or in processing facilities, she said, and a supplier selling a product like bagged lettuce to restaurants may also be stocking grocery stores with it.

Why has it taken so long to identify a source?

It can take a while for people to develop symptoms and get diagnosed. That means public health investigators are often asking people what they ate during the weeks before they became ill, which can be difficult for people to recall, Dr. Bagdasarian said.

Cyclospora is also a “tricky” organism to study, said Kalmia Kniel, a microbiologist at the University of Delaware. It’s challenging to detect in food or water. And compared with other pathogens like bacteria, it’s harder to link cases from a common source using genetic analysis.

The current surge in infections may also be a result of multiple outbreaks happening simultaneously, Dr. Kniel said. The C.D.C. said that it was investigating other cyclosporiasis cases nationally that were unrelated to those linked to Taco Bell in five states.

How risky is fresh produce?

Some health experts are recommending that people take extra precautions with produce that has been implicated in past outbreaks.

Whether you choose to avoid fresh produce right now comes down to your own risk tolerance, said Barbara Kowalcyk, director of the Institute for Food Safety and Nutrition Security at George Washington University. She said that she was avoiding higher risk items for the time being. “Quite honestly, I don’t have time in my life for explosive diarrhea,” she said.

Cyclospora isn’t typically life-threatening, but lengthy bouts of diarrhea can cause severe dehydration, which is especially dangerous for those in vulnerable groups, including children, older adults and people who are pregnant or otherwise immunocompromised.

Telling healthy people outside of those groups to avoid entire categories of fresh produce is an “overly broad” recommendation, said Craig Hedberg, a professor of environmental health sciences at the University of Minnesota School of Public Health. Doing so could result in people eating less healthfully and food waste, he said.

Some public health departments are advising people to cook higher-risk produce to at least 158 degrees. This is a more reliable way to kill the parasite than rinsing or freezing.

Health officials in Michigan are advising people to use whole heads of lettuce rather than bagged salad kits, toss the outer two or three layers of leaves and then rinse with water.

In fact, all fresh produce should be rinsed before eating, health officials recommend. Scrub firm fruits and vegetables like melons and cucumbers with a clean brush, and cut away any damaged or bruised areas. When feasible, peel produce before eating.

What are the symptoms?

The most common symptom of cyclosporiasis is watery diarrhea. The illness can also cause a loss of appetite, cramping, bloating, gas, nausea, fatigue and weight loss. Some people may experience a low-grade fever, said Dr. Adam Ressler, an infectious disease physician with Michigan Medicine.

People with symptoms should see a health care provider for testing and treatment, Dr. Ressler said. “I wouldn’t wait too long to get in and get tested,” he said.

Without treatment, symptoms can last anywhere from a few days to a month or more. And they may “wax and wane,” he said.

How is cyclosporiasis treated?

The most effective treatment for cyclosporiasis is Bactrim (a combination oral antibiotic also sold under the brand names Septra and Cotrim). It’s typically taken for seven to 10 days, Dr. Ressler said.

For those who are allergic or intolerant to that medication, a doctor can recommend other treatment options, such as different types of antibiotics, he said.

Untreated, symptoms like chronic diarrhea can last for a month or more, Dr. Ressler said.

Staying hydrated is important, said Dr. Thomas Moore, a clinical professor of medicine at the University of Kansas School of Medicine-Wichita. He recommended making a simple rehydration solution with water, sugar and salt to replace electrolytes and fluids.

Are federal health cuts hampering the response?

It’s possible. State and local health departments rely on federal funds to monitor, detect and respond to food-borne illnesses. In 2026, those agencies received only about 40 percent of what they requested for those efforts, Dr. Kowalcyk said.

During the coronavirus pandemic, state health departments received emergency federal funding to support their response to the virus. Many of those departments used that money to improve their ability to respond to other types of future outbreaks. That stream of funding has since ended, Dr. Kowalcyk said, resulting in many staff and program cuts. 

Last summer, the C.D.C. quietly scaled back its Foodborne Diseases Active Surveillance Network, or FoodNet, which made it optional for the 10 member states to track cyclospora cases and report them to the C.D.C. While those states still have other ways to report infections, experts say that this change may result in many cases going uncounted.

Alice Callahan is a Times reporter covering nutrition and health. She has a Ph.D. in nutrition from the University of California, Davis.