Federal health authorities have declared the largest cyclospora food poisoning outbreak in U.S. history over as questions remain about how the parasite that causes it ended up in our food supply.Nearly 13,000 cases were tied to the multistate outbreak, representing the bulk of an unprecedented surge in the parasitic diarrheal illness this year.Officials from the U.S. Food and Drug Administration expressed confidence Friday that all recalled iceberg lettuce associated with this specific outbreak has been completely removed from the market.Even with the outbreak deemed over, several questions remain, including details about its origin. Because the parasite is spread through human feces, it still is not clear how sewage could have contaminated enough food to sicken so many people.Taylor Farms was identified as the source of the lettuce and the company recalled iceberg lettuce grown in central Mexico (Getty Images)"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, representing a more than sixteenfold increase compared to the number reported during the same period last year. More than 6,100 suspected cases have also been logged. The worst previous year in the U.S. for infections was 2019, when about 4,700 illnesses were reported nationally. While this year’s surge was dominated by an outbreak across 21 states linked to iceberg lettuce, thousands of other cases have not been linked to that outbreak.Michigan was the hardest hit state, recording 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, leading the company to recall iceberg lettuce grown in central Mexico. The Taylor Farms recall involved thousands of packaged salad products combining iceberg lettuce with other vegetables, which were shipped to major U.S. restaurant chains, including Yum Brands, the owner of 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, adding that samples from those inspections are pending analysis. Ultimately, the outbreak has 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.