An outbreak of an intestinal illness that causes diarrhea, nausea and fatigue has been detected in 31 states, according to federal health authorities, but the source is still under investigation.
As of Thursday, the Centers for Disease Control and Prevention said it had received reports of 843 cases of cyclosporiasis, the gastrointestinal affliction caused by the parasite Cyclospora.
But the true number of infections is likely much higher, because that figure only represents cases reported by states directly to the CDC. There is also a lag between symptom onset and reporting, and many people recover from the illness without medical treatment. Michigan alone reported 1,562 cyclosporiasis cases as of Friday.
According to the CDC, as of Thursday, there had been been 86 hospitalizations nationwide and no deaths.
People can contract the illness by eating food or drinking water contaminated with the parasite. Previous outbreaks have been linked to fresh produce. In 2018, McDonald's removed salads from restaurants in 14 states after federal health officials linked them to dozens of cases of cyclosporiasis, and tainted lettuce imported from Mexico was suspected to have sickened 400 people in the U.S. in 2013.
It's typical for cyclosporiasis infections to rise in the spring and summer, but the CDC said Friday that multiple states had reported a larger jump in cases over the previous two weeks than they had during the same period last year.
Where are cyclosporiasis infections occurring?
Health officials from California to Texas to Florida have reported cases of cyclosporiasis since the start of May.
Some of the hardest-hit areas appear to be in the Midwest and Northeast, including Michigan and New York.
The Ohio Department of Health reported 177 cyclosporiasis cases as of July 2, most of which occurred in June. Dr. Bruce Vanderhoff, director of the Ohio Department of Health, said cyclosporiasis is a "serious illness that can cause dehydration and require people to seek emergency medical care, and it should be taken seriously."
According to the CDC, those sickened with the disease have ranged in age from 5 to 88 years old.
The total number of nationwide cases is expected to grow, due to the estimated six-week gap between when illnesses begin and when they are reported to federal health authorities.
What's causing the outbreak?
That is still unclear. The CDC says it is continuing to try to identify the source or sources of the recent surge of cyclosporiasis infections.
Investigators do that in part by interviewing those who've become sick to find out what they've eaten. But since symptoms can appear anywhere between two days and two weeks or more after a person was infected, they may not remember everything they ate during that period.
Previous U.S. outbreaks of cyclosporiasis have been associated with raspberries, basil, cilantro, snow peas and lettuce, according to the Food and Drug Administration.
How to prevent cyclosporiasis
Cooking produce is an effective way to avoid an infection, as heating food to 158 degrees Fahrenheit or higher kills Cyclospora.
Public health officials also suggest that people thoroughly wash all of their fresh produce, including herbs, though the parasites are not easy to rinse off.
It is also important for home cooks to observe standard food safety rules, such as washing their hands with soap and water before and after handling fresh produce.
Anyone who suspects they've been sickened with cyclosporiasis and is experiencing dehydration or severe diarrhea is encouraged to see a doctor. Cyclosporiasis infections are typically treated with antibiotics.
NPR's Allison Aubrey contributed reporting.
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