Cyclospora outbreaks start with human contamination. Nobody has ever been able to figure out how

As cases of explosive diarrhea continue to rise across the US this summer, health officials are grappling with a longstanding question about the tiny parasite that’s behind all this torment: Just how the heck is poop getting into our food?

The appalling truth about cyclospora is that humans are its lone source. It’s only ever been found in human feces. So if it’s in our food or water, “it doesn’t take a rocket scientist or a microbiologist to overthink this,” said Dr. Mark Moorman, who recently retired as director of the Office of Food Safety at the US Food and Drug Administration. “Humans are the source.” And here’s another unsettling thing about cyclospora: Investigators have never definitively identified the cause of an outbreak. In several cases, they’ve found some strong suggestive evidence that puts cyclospora close to produce growing or processing, but they’ve never identified how the poop is entering the agricultural system in the first place. That question is becoming more pressing, however, as the US contends with its largest ever cyclospora caseload, which has swelled to more than 11,500 confirmed and suspected cases across 41 states, according to the CDC. These are cases that were acquired in the US, rather than from travel abroad, which makes this year unusual. “Typically, we only see 2,000 to 4,000 a year, so this is a pretty big spike,” said Dr. Keith Schneider, a professor of food safety at the University of Florida. Generally, about 1 in 5 is acquired during international travel. Cases of cyclospora tend to rise during the summer months. This year, the FDA has launched investigations into six outbreaks. The largest of these is a nine-state outbreak in the Midwest believed to be linked to iceberg lettuce grown in Mexico. The FDA recently announced that it is looking into another cluster of 72 related cases that doesn’t yet have a known source. “We’ve not seen anything like this,” said Dr. Kali Kniel, a professor of microbial food safety at the University of Delaware. Typically, the US might investigate one or two outbreaks of cyclosporiasis each year. “I think we’re looking at an unusual year with multiple outbreaks happening at the same time.”