More than 200 people have been infected by an intestinal parasite after reportedly eating vegetables from Fresh Del Monte Produce vegetable trays, federal authorities said.

The Centers for Disease Control and Prevention reported there were 212 cases of the infection, cyclosporiasis, in Iowa, Michigan, Minnesota and Wisconsin as of Thursday.

Those infected reported eating from the prepackaged vegetable trays, which included broccoli, cauliflower, carrots and dill dip. Most of the trays were bought from Kwik Trip or Kwik Star convenience stores in those states, according to the CDC.

Del Monte and Kwik Trip could not be immediately reached for comment on Saturday.

Outbreaks of cyclosporiasis have been linked to imported fresh produce contaminated with a microscopic parasite called Cyclospora cayetanensis. The infection first gained prominence during an outbreak in the mid-1990s and has shown up nearly every year since, the CDC said.

The infection can cause a host of stomach-related illnesses, fever and fatigue, and symptoms typically show up one week after the contaminated food was consumed. That means that the number of cases could continue to climb.

It is also one of the main reasons that cyclosporiasis is so difficult to understand, said Michael Osterholm, a professor at the University of Minnesota and an international food-borne disease expert.

“By the time cases are detected, the product is long gone,” he said. “It’s very hard to trace back.”

He said he suspected the number of cases was much higher than the 212 confirmed so far by health officials.

In 1996, more than 1,000 people were sickened by cyclospora parasites, catching health officials off guard and prompting them to ramp up food testing to try to trace the source of the outbreak.

Since then, cyclospora-related outbreaks have been linked to raspberries, basil, snow peas, sugar snap peas, cilantro and cabbage. In 2013, more than 600 cases of cyclosporiasis in two dozen states were tied to a salad mix.

Osterholm, director of the university’s Center for Infectious Disease Research and Policy, said that the prevalence of the infection had been tied to an increase in imported produce from South and Central America and Mexico, but that the source of the infection remained unclear.

According to the CDC, the parasite most likely got on the produce through feces. But Osterholm said it still is not known exactly how and why the produce gets contaminated.

“We have to find out what it is,” he said. “Is it wildlife?”

The outbreaks typically happen in May and June, which could indicate that they are tied to an animal’s life cycle.

“We’re not any farther along in preventing them today than we were back then,” he said.

It’s unclear which vegetable in the trays is behind the current outbreak.

Last month, Del Monte recalled three of its products in stores in six states in the Midwest: six-, 12- and 28-ounce vegetable trays that had broccoli, cauliflower, celery, carrots and dill dip. All those products had a “best if enjoyed by” date of June 17.