Nearly 7,000 people nationwide may have cyclosporiasis, a foodborne illness that can cause weeks of severe diarrhea, the Centers for Disease Control and Prevention said Tuesday. The bulk of the cases are in Michigan, which has confirmed 3,309 cases on its own.

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In a health alert, the CDC reported it had confirmed 1,645 people nationwide have been sickened by cyclosporiasis. An additional 5,100 cases are under investigation.

At least 34 states are reporting cases. The national tally lags behind what states are reporting as the agency must confirm each case.

According to the CDC, 141 people have been hospitalized since cases were first reported in May. No deaths have been reported.

There have been 403 cases in New York City; 374 of them were reported between May and July 10, according to the city’s health department.

The CDC suspects that at least 400 of the cases across four states — Kentucky, Michigan, Ohio and West Virginia — are connected.

Michigan health officials said that based on more than 1,000 interviews with people who’ve tested positive, lettuce or salad greens have emerged as possible sources.

Health officials haven’t identified a supplier of the produce or where the products have been distributed. Michigan’s chief medical executive, Dr. Natasha Bagdasarian, suggested the source of the outbreak is likely larger than one restaurant chain or grocer.

“We know that contamination usually occurs where the produce was grown or processed, and because distributors supply the same produce to multiple types of establishments,” she said, “it’s unlikely for there to be one single place as the exposure for all of our cases.”

Nationally, no single product, restaurant, grocer or distributor has been implicated as the source of the illnesses and there have been no recalls.

The Food and Drug Administration said last week that it’s launched an investigation.

Cyclosporiasis is a foodborne illness caused by cyclospora, a microscopic parasite. That parasite can spread from human feces to products like lettuce through contaminated water or unsafe food handling.

Previous outbreaks have involved raspberries, bagged lettuce or salads, cilantro, basil, green onions and snow peas.

Once the parasites latch on, they’re hard to wash away. There’s no evidence that soaking food or dousing it with vinegar or commercial vegetable washes is enough to get rid of them. They need to be scrubbed off or cut off as much as possible.

If people eat food contaminated with cyclospora, it takes up to two weeks for people to get sick. The illness can begin with flu-like symptoms, such as fatigue, body aches and nausea.

The hallmark symptom of cyclosporiasis is severe, watery diarrhea that’s sometimes uncontrollable. Cyclosporiasis is most commonly treated with Bactrim, an antibiotic.

Left untreated, symptoms can linger for weeks. People with the illness need to drink plenty of fluids to counter the diarrhea.