At least 20 people across 17 states have been hospitalized due to severe gastrointestinal illness as more than 1,000 cases of the parasitic infection cyclosporiasis have been reported across the country, the U.S. Centers for Disease Control and Prevention (CDC) reports.

The CDC and the Food and Drug Administration (FDA) are investigating several clusters of the illness in Illinois, New York, Pennsylvania and Texas, according to CNN. However, the worst-hit state so far is Michigan, with 1,251 cases reported as of July 9, the Michigan Department of Health and Human Services (MDHHS) reported. According to the Associated Press (AP), this marks the biggest cyclosporiasis outbreak in the state's history.

Cyclosporiasis is a gastrointestinal illness caused by the protozoan parasite Cyclospora cayetanensis, which can enter the body when someone consumes food or water contaminated with the parasite. It then infects the small intestines, invading epithelial cells that line the gut, causing inflammation and leading to watery and often "explosive" diarrhea, according to the CDC. Symptoms usually kick in around a week after infection and can last over a month without antibiotic treatment. While the illness can go away on its own, it is also common for symptoms to disappear for a bit and then return.

Cyclosporiasis is not usually life-threatening, but it can cause severe illness and dehydration, particularly in young children, older adults and people with compromised immune systems. In rare cases, the condition may lead to complications like biliary disease, in which the flow of bile gets blocked and impacts the liver and digestive health, according to the MDHHS.

As of July 1, there was no evidence of a single, multistate outbreak of cyclosporiasis, but the CDC is investigating several clusters. According to an email from the health agency, many of the clusters appear to be linked to Mexican-style restaurants, a chain of grocery stores and a catered event, CNN reported. The FDA is conducting trace-back investigations on white and green onions, cucumbers and cilantro, according to CNN.

Meanwhile, the MDHHS warned that, while the source of the current outbreak is still uncertain, previous Cyclospora outbreaks have been linked to contaminated fresh produce, including raspberries, basil and bagged salad.

Cyclosporiasis cases tend to rise during the spring and summer, with May 1 to Aug. 31 considered "cyclosporiasis season," according to the CDC. However, there have been more cases than usual this year: Whereas Michigan typically reports 50 cases over a whole year, the state has seen over 1,000 cases in just a few weeks, the AP reported.

Compared with outbreaks of E. coli and Salmonella, clusters of Cyclospora are much harder to trace back to their source. This is because the parasite can reproduce sexually, meaning its genetic material is mixed up with each generation, according to Charles River Laboratories. However, Dr. Natasha Bagdasarian, Michigan's chief medical executive, told the AP that "there is clearly a linked outbreak happening right now."

To minimize your risk of becoming infected with Cyclospora, wash all fruits and vegetables thoroughly before eating them; cut away any bruised or damaged areas; and refrigerate cut, peeled or cooked produce as soon as possible, the Michigan Department of Agriculture and Rural Development recommends. If you are experiencing sudden gastrointestinal issues, health officials suggest you visit your local health care provider.