The cyclospora food poisoning outbreak linked to recalled iceberg lettuce is over, U.S. health officials said Friday.

The Centers for Disease Control and Prevention said there is no longer a risk of people getting sick with cyclosporiasis from this outbreak and is confident that all related iceberg lettuce is off the market.

The U.S. Food and Drug Administration said that while the outbreak is over, it will continue its investigation of cyclospora infections linked to the iceberg lettuce sourced from central Mexico and recalled by Taylor Farms de Mexico.

“FDA will continue to work with federal, state, and international partners to complete its investigation and will provide updates as they become available,” the agency said on Friday.

Nearly 13,000 reported cyclospora cases were tied to the outbreak spanning 21 states, federal officials say. They represent the bulk of an unprecedented surge in the parasitic diarrheal illness this year. More than 570 people have been hospitalized and two died, according to the FDA and CDC, as of Friday.

Even with the outbreak deemed over, several questions remain, including about its origin. The parasite is spread through human feces, and it still isn’t clear how sewage could have contaminated enough food to sicken so many people.

“This contamination can happen if produce is grown in soil or comes into contact with water that has been contaminated,” said Ellen Shumaker, director of outreach for a North Carolina State University food safety program, Safe Plates.

Since May 1, the government has received reports of nearly 20,000 lab-confirmed cases, or more than 16 times the number reported during the same period last year. More than 6,100 suspected cases also have been reported.

The worst previous year in the U.S. for infections was 2019, when about 4,700 illnesses were reported nationally.

This year’s surge in cases was dominated by an outbreak in 21 states linked to iceberg lettuce. But there are thousands of other cases that have not been linked to that outbreak.

Michigan was the hardest-hit state, with the most reported cases and the two U.S. deaths tied to the outbreak.

Cyclospora is a microscopic, spherical parasite that commonly causes watery diarrhea “with frequent and sometimes explosive bowel movements,” according to the CDC. Outbreaks tend to occur most often in the late spring and summer.

The illness, called cyclosporiasis, is less common than foodborne illnesses caused by other germs, including salmonella and E. coli. Many cases are never linked to a specific food or other source.

In this outbreak, investigators initially zeroed in on lettuce served at Taco Bell. Federal officials subsequently focused on Taylor Farms as the source of the lettuce, and the company recalled iceberg lettuce grown in central Mexico.

The Taylor Farms recall involved thousands of packaged salad products combining iceberg lettuce with other vegetables. The products were shipped to major U.S. restaurant chains, including Yum Brands, which owns Taco Bell, Pizza Hut and KFC.

The FDA has ended its on-site inspections and sample collection at iceberg lettuce growers and the processing facility in Mexico, the agency said Friday. Samples from the on-site inspections are pending analysis, the FDA said.

The outbreak cast a spotlight on a worsening problem: U.S. regulators are conducting fewer international inspections to catch contaminated produce before it lands on American plates.