NEW YORK — Nearly 1,000 people in Michigan have been diagnosed with a parasitic infection that can cause weeks of watery diarrhea, making it the largest such outbreak in the state’s history and one of the largest in the country in years.

No deaths have been reported and the source of the cyclospora infections has not been identified. Meanwhile, investigations into similar illnesses have been conducted in 28 other states, including Ohio, where people across the border in Michigan are also getting sick.

Michigan officials first announced the outbreak last week, when they were aware of more than 170 cases, all in the southeast corner of the state, since June 22. Michigan typically identifies only about 50 cases each year.

On Wednesday, the state reported the number had risen to 992, including about 40 hospitalizations. Across the state line, Lucas County, Ohio, reported 306 cases as of Wednesday. Northwest Ohio has seen more than 400 cases.

Cyclospora surges can be difficult to investigate and sources of food poisoning can be difficult to establish. But “there is clearly a related outbreak right now,” Dr. Natasha Bagdasarian, Michigan’s chief medical officer, told The Associated Press on Wednesday.

Here’s what you should know about the current situation:

Cyclospora is a microscopic, spherical parasite that commonly causes watery diarrhea “with frequent and sometimes explosive stools,” according to the U.S. Centers for Disease Control and Prevention. The disease, called cyclosporiasis, is not usually life-threatening and is usually treated with antibiotics. Outbreaks tend to occur most frequently in late spring and summer.

The heat-loving parasite infects the intestines and spreads through feces. In the past, people became infected by consuming fruits or vegetables that were exposed to irrigation water contaminated with feces.

It is less common than other types of foodborne illnesses, including salmonella and E. coli. For years, few cyclospora outbreaks were reported each year in the United States. But the number began rising about a decade ago, with a particularly notable increase in 2018 and 2019. Experts attribute the increases to climate change and better detection.

Complete data on cyclospora outbreaks are lacking. But available information shows that only a small number of documented outbreaks in the last 20 years have exceeded 1,000 cases. That short list includes a 1997 outbreak linked to Guatemalan raspberries that sickened more than 1,000 in the United States and Canada, and a 2019 outbreak linked to Mexican basil that sickened more than 2,400.

There are several reasons why it’s difficult to know the exact number of victims, said Melanie Firestone, a foodborne illness researcher at the University of Minnesota. Some tests used to detect types of food poisoning are not designed to detect cyclospora, “so there is a lot of underreporting when it comes to this,” he said.

Other challenges: Technicians can’t grow the parasite in labs, making it difficult to extract evidence from contaminated products. And it can be difficult to determine what food-ill people had in common, because sometimes it’s a single ingredient that may be common in several recipes, such as basil or cilantro.

Additionally, food distributors may funnel contaminated food to both grocery stores and restaurants, making it difficult to discern where the contaminated food comes from. Investigations can take months and sometimes never find a clear source.

Cases appear to be increasing in and around southeastern Michigan. But it is not considered a national health emergency.

There is no evidence that the parasite has evolved to become more infectious, said Dianna Blau, acting chief of the CDC’s parasitic diseases section.

Thousands of cyclospora illnesses are reported each year in the U.S., and it’s not yet clear how unusual this year will be, he added. That said, total cases so far are four times higher than at the same time last year, according to the CDC’s current national data, which lags far behind what states are reporting.

Michigan appears to be suffering the worst, but the state’s aggressiveness in investigating and reporting cases may be “part of the reason why this seems like a Michigan problem,” Bagdasarian said.

People who have diarrhea that does not go away on its own within a few days should see a health care provider and be tested for the possibility of cyclospora, officials say.

The best way to prevent infection from a parasite is to avoid food or water that may have been contaminated.

Fresh produce should be washed thoroughly before consumption. But keep in mind that cyclospora can actually stick to some foods, so washing may not eliminate the risk of infection.

While Michigan officials investigate the potential source, they are recommending consumers buy whole heads of lettuce instead of pre-washed bagged lettuces or salad mixes, and remove the outer two or three leaves before washing the remaining leaves under running water.

They also say to cook vegetables when possible.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.